Picture decoding device, picture decoding method, and picture decoding program with history-based candidate selection

ABSTRACT

Technology for improving coding efficiency by performing a block split suitable for picture coding and decoding is provided. A picture decoding device includes a spatial candidate derivation unit configured to derive a spatial candidate from inter prediction information of a block neighboring a decoding target block and register the derived spatial candidate as a candidate in a first candidate list, a history-based candidate derivation unit configured to generate a second candidate list by adding a history-based candidate included in a history-based candidate list as a candidate to the first candidate list, a candidate selection unit configured to select a selection candidate from candidates included in the second candidate list; and an inter prediction unit configured to perform inter prediction using the selection candidate. The history-based candidate derivation unit switches between whether or not a history-based candidate overlapping a candidate included in the first candidate list is added in accordance with a prediction mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage entry of PCT Application No:PCT/JP2019/050093 filed Dec. 20, 2019, which claims priority to JapanesePatent Application Nos. 2018-247402 filed Dec. 28, 2018 and 2019-171784filed Sep. 20, 2019, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to picture coding and decoding technologyfor dividing a picture into blocks and performing prediction.

In picture coding and decoding, a target picture is divided into blocks,each of which is a set of a prescribed number of samples, and a processis performed in units of blocks. Coding efficiency is improved bydividing a picture into appropriate blocks and appropriately settingintra picture prediction (intra prediction) and inter picture prediction(inter prediction).

In moving-picture coding/decoding, coding efficiency is improved byinter prediction for performing prediction from a coded/decoded picture.Patent Literature 1 describes technology for applying an affinetransform at the time of inter prediction. It is not uncommon for anobject to cause deformation such as enlargement/reduction and rotationin moving pictures and efficient coding is enabled by applying thetechnology of Patent Literature 1.

PRIOR ART DOCUMENTS Patent Documents Patent Document 1

-   Japanese Unexamined Patent Application, First Publication No.    H9-172644

SUMMARY OF THE INVENTION

However, because the technology of Patent Literature 1 involves apicture transform, there is a problem that the processing load is great.In view of the above problem, the present invention provides efficientcoding technology with a low load.

For example, embodiments to be described below disclose the followingaspects.

There is provided a picture decoding device including: a spatialcandidate derivation unit configured to derive a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and register the derived spatial candidate as a candidate in afirst candidate list; a history-based candidate derivation unitconfigured to generate a second candidate list by adding a history-basedcandidate included in a history-based candidate list as a candidate tothe first candidate list; a candidate selection unit configured toselect a selection candidate from candidates included in the secondcandidate list; and an inter prediction unit configured to perform interprediction using the selection candidate, wherein the history-basedcandidate derivation unit switches between whether or not ahistory-based candidate overlapping a candidate included in the firstcandidate list is added in accordance with a prediction mode.

In the picture decoding device, the prediction mode is a merge mode anda motion vector predictor mode, the candidate when the prediction modeis the merge mode is motion information, and the candidate when theprediction mode is the motion vector predictor mode is a motion vector.

In the picture decoding device, the history-based candidate derivationunit adds the history-based candidate as a candidate to the firstcandidate list if the history-based candidate does not overlap acandidate included in the first candidate list when the prediction modeis the merge mode and adds the history-based candidate as a candidate tothe first candidate list regardless of whether or not the history-basedcandidate overlaps a candidate included in the first candidate list whenthe prediction mode is the motion vector predictor mode.

The picture decoding device further includes a history-based motionvector predictor candidate list update unit configured to update ahistory-based motion vector predictor candidate list with the selectioncandidate so that the history-based motion vector predictor candidatelist does not include an overlapping candidate when the prediction modeis the merge mode and update the history-based motion vector predictorcandidate list with motion information including at least the selectioncandidate and a reference index indicating a picture for referring tothe selection candidate so that the history-based motion vectorpredictor candidate list does not include an overlapping candidate whenthe prediction mode is the motion vector predictor mode.

In the picture decoding device, the maximum number of candidatesincluded in the candidate list when the prediction mode is the mergemode is larger than the maximum number of candidates included in thecandidate list when the prediction mode is the motion vector predictormode.

In the picture decoding device, the history-based motion vectorpredictor candidate derivation unit adds the history-based candidate asa candidate to the first candidate list regardless of whether or not thehistory-based candidate overlaps a candidate included in the firstcandidate list if a reference index of the history-based candidate isthe same as a reference index of the decoding target picture when theprediction mode is the motion vector predictor mode.

There is provided a picture decoding method for use in a picturedecoding device, the picture decoding method including steps of:deriving a spatial candidate from inter prediction information of ablock neighboring a decoding target block and registering the derivedspatial candidate as a candidate in a first candidate list; generating asecond candidate list by adding a history-based candidate included in ahistory-based candidate list as a candidate to the first candidate listand switching between whether or not a history-based candidateoverlapping a candidate included in the first candidate list is added inaccordance with a prediction mode; selecting a selection candidate fromcandidates included in the second candidate list; and performing interprediction using the selection candidate.

There is provided a computer program stored in a computer-readablenon-transitory storage medium in a picture decoding device, the computerprogram including instructions for causing a computer of the picturedecoding device to execute steps of: deriving a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and registering the derived spatial candidate as a candidate in afirst candidate list; generating a second candidate list by adding ahistory-based candidate included in a history-based candidate list as acandidate to the first candidate list and switching between whether ornot a history-based candidate overlapping a candidate included in thefirst candidate list is added in accordance with a prediction mode;selecting a selection candidate from candidates included in the secondcandidate list; and performing inter prediction using the selectioncandidate.

There is provided a picture decoding device including: a spatialcandidate derivation unit configured to derive a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and register the derived spatial candidate as a candidate in afirst candidate list; a history-based candidate derivation unitconfigured to generate a second candidate list by adding a history-basedcandidate included in a history-based candidate list as a candidate tothe first candidate list; a candidate selection unit configured toselect a selection candidate from candidates included in the secondcandidate list; and an inter prediction unit configured to perform interprediction using the selection candidate, wherein the history-basedcandidate derivation unit switches between whether or not ahistory-based candidate overlapping a candidate included in the firstcandidate list is added in accordance with a prediction mode, andwherein the prediction mode is a merge mode and a motion vectorpredictor mode, the candidate when the prediction mode is the merge modeis motion information, and the candidate when the prediction mode is themotion vector predictor mode is a motion vector.

In the picture decoding device, the history-based candidate derivationunit adds the history-based candidate as a candidate to the firstcandidate list if the history-based candidate does not overlap acandidate included in the first candidate list when the prediction modeis the merge mode and adds the history-based candidate as a candidate tothe first candidate list regardless of whether or not the history-basedcandidate overlaps a candidate included in the first candidate list whenthe prediction mode is the motion vector predictor mode.

The picture decoding device further includes a history-based motionvector predictor candidate list update unit configured to update ahistory-based motion vector predictor candidate list with the selectioncandidate so that the history-based motion vector predictor candidatelist does not include an overlapping candidate when the prediction modeis the merge mode and update the history-based motion vector predictorcandidate list with motion information including at least the selectioncandidate and a reference index indicating a picture for referring tothe selection candidate so that the history-based motion vectorpredictor candidate list does not include an overlapping candidate whenthe prediction mode is the motion vector predictor mode.

In the picture decoding device, the maximum number of candidatesincluded in the candidate list when the prediction mode is the mergemode is larger than the maximum number of candidates included in thecandidate list when the prediction mode is the motion vector predictormode.

In the picture decoding device, the history-based motion vectorpredictor candidate derivation unit adds the history-based candidate asa candidate to the first candidate list regardless of whether or not thehistory-based candidate overlaps a candidate included in the firstcandidate list if a reference index of the history-based candidate isthe same as a reference index of the decoding target picture when theprediction mode is the motion vector predictor mode.

There is provided a picture decoding method including steps of: derivinga spatial candidate from inter prediction information of a blockneighboring a decoding target block and registering the derived spatialcandidate as a candidate in a first candidate list; generating a secondcandidate list by adding a history-based candidate included in ahistory-based candidate list as a candidate to the first candidate listand switching between whether or not a history-based candidateoverlapping a candidate included in the first candidate list is added inaccordance with a prediction mode; selecting a selection candidate fromcandidates included in the second candidate list; and performing interprediction using the selection candidate, wherein the prediction mode isa merge mode and a motion vector predictor mode, the candidate when theprediction mode is the merge mode is motion information, and thecandidate when the prediction mode is the motion vector predictor modeis a motion vector.

There is provided a computer program stored in a computer-readablenon-transitory storage medium in a picture decoding device, the computerprogram including instructions for causing a computer of the picturedecoding device to execute steps of: deriving a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and registering the derived spatial candidate as a candidate in afirst candidate list; generating a second candidate list by adding ahistory-based candidate included in a history-based candidate list as acandidate to the first candidate list, switching between whether or nota history-based candidate overlapping a candidate included in the firstcandidate list is added in accordance with a prediction mode, adding thehistory-based candidate as a candidate to the first candidate list ifthe history-based candidate does not overlap a candidate included in thefirst candidate list when the prediction mode is a merge mode, theprediction mode being the merge mode and a motion vector predictor mode,the candidate when the prediction mode is the merge mode being motioninformation, the candidate when the prediction mode is the motion vectorpredictor mode being a motion vector, and adding the history-basedcandidate as a candidate to the first candidate list regardless ofwhether or not the history-based candidate overlaps a candidate includedin the first candidate list when the prediction mode is the motionvector predictor mode; selecting a selection candidate from candidatesincluded in the second candidate list; and performing inter predictionusing the selection candidate.

The above description is an example. The scopes of the presentapplication and the present invention are not limited or restricted bythe above description. Also, it should be understood that thedescription of the “present invention” in the present specification doesnot limit the scope of the present application or the present inventionand is used as an example.

According to the present invention, it is possible to implement a highlyefficient picture coding/decoding process with a low load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a picture coding device according to anembodiment of the present invention.

FIG. 2 is a block diagram of a picture decoding device according to anembodiment of the present invention.

FIG. 3 is an explanatory flowchart showing an operation of dividing atree block.

FIG. 4 is a diagram showing a state in which an input picture is dividedinto tree blocks.

FIG. 5 is an explanatory diagram showing Z-scan.

FIG. 6A is a diagram showing a divided shape of a block.

FIG. 6B is a diagram showing a divided shape of a block.

FIG. 6C is a diagram showing a divided shape of a block.

FIG. 6D is a diagram showing a divided shape of a block.

FIG. 6E is a diagram showing a divided shape of a block.

FIG. 7 is an explanatory flowchart showing an operation of dividing ablock into four parts.

FIG. 8 is an explanatory flowchart showing an operation of dividing ablock into two or three parts.

FIG. 9 is syntax for expressing a shape of block split.

FIG. 10A is an explanatory diagram showing intra prediction.

FIG. 10B is an explanatory diagram showing intra prediction.

FIG. 11 is an explanatory diagram showing a reference block of interprediction.

FIG. 12 is syntax for expressing a coding block prediction mode.

FIG. 13 is a diagram showing correspondence between a syntax elementrelated to inter prediction and a mode.

FIG. 14 is an explanatory diagram showing affine motion compensation oftwo control points.

FIG. 15 is an explanatory diagram showing affine motion compensation ofthree control points.

FIG. 16 is a block diagram of a detailed configuration of an interprediction unit 102 of FIG. 1 .

FIG. 17 is a block diagram of a detailed configuration of a normalmotion vector predictor mode derivation unit 301 of FIG. 16 .

FIG. 18 is a block diagram of a detailed configuration of a normal mergemode derivation unit 302 of FIG. 16 .

FIG. 19 is an explanatory flowchart showing a normal motion vectorpredictor mode derivation process of the normal motion vector predictormode derivation unit 301 of FIG. 16 .

FIG. 20 is a flowchart showing a processing procedure of the normalmotion vector predictor mode derivation process.

FIG. 21 is an explanatory flowchart showing a processing procedure of anormal merge mode derivation process.

FIG. 22 is a block diagram of a detailed configuration of an interprediction unit 203 of FIG. 2 .

FIG. 23 is a block diagram of a detailed configuration of a normalmotion vector predictor mode derivation unit 401 of FIG. 22 .

FIG. 24 is a block diagram of a detailed configuration of a normal mergemode derivation unit 402 of FIG. 22 .

FIG. 25 is an explanatory flowchart showing a normal motion vectorpredictor mode derivation process of the normal motion vector predictormode derivation unit 401 of FIG. 22 .

FIG. 26 is an explanatory diagram showing a processing procedure ofinitializing/updating a history-based motion vector predictor candidatelist.

FIG. 27 is a flowchart of an identical element checking processingprocedure in the processing procedure of initializing/updating ahistory-based motion vector predictor candidate list.

FIG. 28 is a flowchart of an element shift processing procedure in theprocessing procedure of initializing/updating a history-based motionvector predictor candidate list.

FIG. 29 is an explanatory flowchart showing a history-based motionvector predictor candidate derivation processing procedure.

FIG. 30 is an explanatory flowchart showing a history-based mergingcandidate derivation processing procedure.

FIG. 31A is an explanatory diagram showing an example of a history-basedmotion vector predictor candidate list update process.

FIG. 31B is an explanatory diagram showing an example of a history-basedmotion vector predictor candidate list update process.

FIG. 31C is an explanatory diagram showing an example of a history-basedmotion vector predictor candidate list update process.

FIG. 32 is an explanatory diagram showing motion-compensated predictionwhen a clock time of a reference picture (RefL0Pic) of L0 is earlierthan that of a target picture (CurPic) as L0-prediction.

FIG. 33 is an explanatory diagram showing motion-compensated predictionwhen a clock time of a reference picture of L0-prediction is later thanthat of a target picture as L0-prediction.

FIG. 34 is an explanatory diagram showing a prediction direction ofmotion-compensated prediction when a clock time of a reference pictureof L0-prediction is earlier than that of a target picture and a clocktime of a reference picture of L1-prediction is later than that of atarget picture as bi-prediction.

FIG. 35 is an explanatory diagram showing a prediction direction ofmotion-compensated prediction when a clock time of a reference pictureof L0-prediction and a clock time of a reference picture ofL1-prediction are earlier than that of a target picture asbi-prediction.

FIG. 36 is an explanatory diagram showing a prediction direction ofmotion-compensated prediction when a clock time of a reference pictureof L0-prediction and a clock time of a reference picture ofL1-prediction are later than that of a target picture as bi-prediction.

FIG. 37 is an explanatory diagram showing an example of a hardwareconfiguration of a coding/decoding device according to an embodiment ofthe present invention.

FIG. 38 is a table showing an example of history-based motion vectorpredictor candidates added according to initialization of ahistory-based motion vector predictor candidate list.

FIG. 39 is a table showing another example of history-based motionvector predictor candidates added according to initialization of ahistory-based motion vector predictor candidate list.

FIG. 40 is a table showing another example of history-based motionvector predictor candidates added according to initialization of ahistory-based motion vector predictor candidate list.

FIG. 41 is a flowchart for describing a history-based motion vectorpredictor candidate derivation processing procedure of modified example2 of the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Technology and technical terms used in the embodiment will be defined.

<Tree Block>

In the embodiment, a coding/decoding target picture is equally dividedinto units of a predetermined size. This unit is defined as a treeblock. Although the size of the tree block is 128×128 samples in FIG. 4, the size of the tree block is not limited thereto and any size may beset. The tree block of a target (corresponding to a coding target in acoding process or a decoding target in the decoding process) is switchedin a raster scan order, i.e., from left to right and from top to bottom.The inside of each tree block can be further recursively divided. Ablock which is a coding/decoding target after the tree block isrecursively divided is defined as a coding block. Also, a tree block anda coding block are collectively defined as blocks. Efficient coding isenabled by performing appropriate block split. The tree block size maybe a fixed value predetermined by the coding device and the decodingdevice or the tree block size determined by the coding device may beconfigured to be transmitted to the decoding device. Here, a maximumsize of the tree block is 128×128 samples and a minimum size of the treeblock is 16×16 samples. Also, a maximum size of the coding block is64×64 samples and a minimum size of the coding block is 4×4 samples.

<Prediction Mode>

Switching is performed between intra prediction (MODE_INTRA) in whichprediction is performed from a processed picture signal of the targetpicture and inter prediction (MODE_INTER) in which prediction isperformed from a picture signal of a processed picture in units oftarget coding blocks.

The processed picture is used for a picture, a picture signal, a treeblock, a block, a coding block, and the like obtained by decoding asignal completely coded in the coding process and is used for a picture,a picture signal, a tree block, a block, a coding block, and the likeobtained by completing decoding in a decoding process.

The mode in which the intra prediction (MODE_INTRA) and the interprediction (MODE_INTER) are identified is defined as the prediction mode(PredMode). The prediction mode (PredMode) has intra prediction(MODE_INTRA) or inter prediction (MODE_INTER) as a value.

<Inter Prediction>

In inter prediction in which prediction is performed from a picturesignal of a processed picture, a plurality of processed pictures can beused as reference pictures. In order to manage a plurality of referencepictures, two types of reference lists of L0 (reference list 0) and L1(reference list 1) are defined and a reference picture is identifiedusing each reference index. In a P slice, L0-prediction (Pred_L0) can beused. In a B slice, L0-prediction (Pred_L0), L1-prediction (Pred_L1),and bi-prediction (Pred_BI) can be used. The L0-prediction (Pred_L0) isinter prediction that refers to a reference picture managed in L0 andthe L1-prediction (Pred_L1) is inter prediction that refers to areference picture managed in L1. The bi-prediction (Pred_BI) is interprediction in which both the L0-prediction and the L1-prediction areperformed and one reference picture managed in each of L0 and L1 isreferred to. Information for identifying the L0-prediction, theL1-prediction, and the bi-prediction is defined as an inter predictionmode. In the subsequent processing, constants and variables with thesubscript LX in the output are assumed to be processed for each of L0and L1.

<Motion Vector Predictor Mode>

The motion vector predictor mode is a mode for transmitting an index foridentifying a motion vector predictor, a motion vector difference, aninter prediction mode, and a reference index and determining interprediction information of a target block. The motion vector predictor isderived from a motion vector predictor candidate derived from aprocessed block neighboring the target block or a block located at thesame position as or in the vicinity of (near) the target block amongblocks belonging to the processed picture and an index for identifying amotion vector predictor.

<Merge Mode>

The merge mode is a mode in which inter prediction information of atarget block is derived from inter prediction information of a processedblock neighboring a target block or a block located at the same positionas or in the vicinity of (near) the target block among blocks belongingto the processed picture without transmitting a motion vector differenceand a reference index.

The processed block neighboring the target block and the interprediction information of the processed block are defined as spatialmerging candidates. The block located at the same position as or in thevicinity of (near) the target block among the blocks belonging to theprocessed picture and inter prediction information derived from theinter prediction information of the block are defined as temporalmerging candidates. Each merging candidate is registered in a mergingcandidate list, and a merging candidate used for prediction of a targetblock is identified by a merge index.

<Neighboring Block>

FIG. 11 is an explanatory diagram showing a reference block that isreferred to in deriving inter prediction information in the motionvector predictor mode and the merge mode. A0, A1, A2, B0, B1, B2, and B3are processed blocks neighboring the target block. TO is a block locatedat the same position as or in the vicinity of (near) the target block inthe target picture among blocks belonging to the processed picture.

A1 and A2 are blocks located on the left side of the target coding blockand neighboring the target coding block. B1 and B3 are blocks located onthe upper side of the target coding block and neighboring the targetcoding block. A0, B0, and B2 are blocks located at the lower left, upperright, and upper left of the target coding block, respectively.

Details of how to handle neighboring blocks in the motion vectorpredictor mode and the merge mode will be described below.

<Affine Motion Compensation>

The affine motion compensation is a process of performing motioncompensation by dividing a coding block into subblocks of apredetermined unit and individually determining a motion vector for eachof the subblocks into which the coding block is divided. The motionvector of each subblock is derived on the basis of one or more controlpoints derived from inter prediction information of a processed blockneighboring the target block or a block located at the same position asor in the vicinity of (near) the target block among blocks belonging tothe processed picture. Although the size of the subblock is 4×4 samplesin the present embodiment, the size of the subblock is not limitedthereto and a motion vector may be derived in units of samples.

An example of affine motion compensation in the case of two controlpoints is shown in FIG. 14 . In this case, the two control points havetwo parameters of a horizontal direction component and a verticaldirection component. Thus, an affine transform in the case of twocontrol points is referred to as a four-parameter affine transform. CP1and CP2 of FIG. 14 are control points.

An example of affine motion compensation in the case of three controlpoints is shown in FIG. 15 . In this case, the three control points havetwo parameters of a horizontal direction component and a verticaldirection component. Thus, an affine transform in the case of threecontrol points is referred to as a six-parameter affine transform. CP1,CP2, and CP3 of FIG. 15 are control points.

Affine motion compensation can be used in both the motion vectorpredictor mode and the merge mode. A mode in which the affine motioncompensation is applied in the motion vector predictor mode is definedas a subblock-based motion vector predictor mode, and a mode in whichthe affine motion compensation is applied in the merge mode is definedas a subblock-based merge mode.

<Inter Prediction Syntax>

The syntax related to inter prediction will be described using FIGS. 12and 13 .

The flag merge_flag in FIG. 12 indicates whether the target coding blockis set to the merge mode or the motion vector predictor mode. The flagmerge_affine_flag indicates whether or not the subblock-based merge modeis applied to the target coding block of the merge mode. The flaginter_affine_flag indicates whether or not to apply the subblock-basedmotion vector predictor mode to the target coding block of the motionvector predictor mode. The flag cu_affine_type_flag is used to determinethe number of control points in the subblock-based motion vectorpredictor mode.

FIG. 13 shows a value of each syntax element and a prediction methodcorresponding thereto. The normal merge mode corresponds to merge_flag=1and merge_affine_flag=0 and is not a subblock-based merge mode. Thesubblock-based merge mode corresponds to merge_flag=1 andmerge_affine_flag=1. The normal motion vector predictor mode correspondsto merge_flag=0 and inter_affine_flag=0. The normal motion vectorpredictor mode is a motion vector predictor merge mode that is not asubblock-based motion vector predictor mode. The subblock-based motionvector predictor mode corresponds to merge_flag=0 andinter_affine_flag=1. When merge_flag=0 and inter_affine_flag=1,cu_affine_type_flag is further transmitted to determine the number ofcontrol points.

<POC>

A picture order count (POC) is a variable associated with a picture tobe coded and is set to a value that is incremented by 1 according to anoutput order of pictures. According to the POC value, it is possible todiscriminate whether pictures are the same, to discriminate ananteroposterior relationship between pictures in the output order, or toderive the distance between pictures. For example, if the POCs of twopictures have the same value, it can be determined that they are thesame picture. When the POCs of two pictures have different values, itcan be determined that the picture with the smaller POC value is thepicture to be output first. A difference between the POCs of the twopictures indicates an inter-picture distance in a time axis direction.

First Embodiment

The picture coding device 100 and the picture decoding device 200according to the first embodiment of the present invention will bedescribed.

FIG. 1 is a block diagram of a picture coding device 100 according tothe first embodiment. The picture coding device 100 according to theembodiment includes a block split unit 101, an inter prediction unit102, an intra prediction unit 103, a decoded picture memory 104, aprediction method determination unit 105, a residual generation unit106, an orthogonal transform/quantization unit 107, a bit strings codingunit 108, an inverse quantization/inverse orthogonal transform unit 109,a decoding picture signal superimposition unit 110, and a codinginformation storage memory 111.

The block split unit 101 recursively divides the input picture togenerate a coding block. The block split unit 101 includes a quad splitunit that divides a split target block in the horizontal direction andthe vertical direction and a binary-ternary split unit that divides thesplit target block in either the horizontal direction or the verticaldirection. The block split unit 101 sets the generated coding block as atarget coding block and supplies a picture signal of the target codingblock to the inter prediction unit 102, the intra prediction unit 103,and the residual generation unit 106. Also, the block split unit 101supplies information indicating a determined recursive split structureto the bit strings coding unit 108. The detailed operation of the blocksplit unit 101 will be described below.

The inter prediction unit 102 performs inter prediction of the targetcoding block. The inter prediction unit 102 derives a plurality of interprediction information candidates from the inter prediction informationstored in the coding information storage memory 111 and the decodedpicture signal stored in the decoded picture memory 104, selects asuitable inter prediction mode from the plurality of derived candidates,and supplies the selected inter prediction mode and a predicted picturesignal according to the selected inter prediction mode to the predictionmethod determination unit 105. A detailed configuration and operation ofthe inter prediction unit 102 will be described below.

The intra prediction unit 103 performs intra prediction of the targetcoding block. The intra prediction unit 103 refers to a decoded picturesignal stored in the decoded picture memory 104 as a reference sampleand generates a predicted picture signal according to intra predictionbased on coding information such as an intra prediction mode stored inthe coding information storage memory 111. In the intra prediction, theintra prediction unit 103 selects a suitable intra prediction mode fromamong a plurality of intra prediction modes and supplies a selectedintra prediction mode and a predicted picture signal according to theselected intra prediction mode to the prediction method determinationunit 105.

Examples of intra prediction are shown in FIGS. 10A and 10B. FIG. 10Ashows the correspondence between a prediction direction of intraprediction and an intra prediction mode number. For example, in intraprediction mode 50, an intra prediction picture is generated by copyingreference samples in the vertical direction. Intra prediction mode 1 isa DC mode and is a mode in which all sample values of the target blockare an average value of reference samples. Intra prediction mode 0 is aplanar mode and is a mode for creating a two-dimensional intraprediction picture from reference samples in the vertical and horizontaldirections. FIG. 10B is an example in which an intra prediction pictureis generated in the case of intra prediction mode 40. The intraprediction unit 103 copies the value of the reference sample in thedirection indicated by the intra prediction mode with respect to eachsample of the target block. When the reference sample of the intraprediction mode is not at an integer position, the intra prediction unit103 determines a reference sample value according to an interpolationfrom reference sample values of neighboring integer positions.

The decoded picture memory 104 stores a decoded picture generated by thedecoding picture signal superimposition unit 110. The decoded picturememory 104 supplies the stored decoded picture to the inter predictionunit 102 and the intra prediction unit 103.

The prediction method determination unit 105 determines the optimumprediction mode by evaluating each of intra prediction and interprediction using coding information, a residual code amount, an amountof distortion between a predicted picture signal and a target picturesignal, and the like. In the case of intra prediction, the predictionmethod determination unit 105 supplies intra prediction information suchas an intra prediction mode as the coding information to the bit stringscoding unit 108. In the case of the inter prediction merge mode, theprediction method determination unit 105 supplies inter predictioninformation such as a merge index and information indicating whether ornot the mode is a subblock-based merge mode (a subblock-based mergeflag) as the coding information to the bit strings coding unit 108. Inthe case of the motion vector predictor mode of inter prediction, theprediction method determination unit 105 supplies inter predictioninformation such as the inter prediction mode, a motion vector predictorindex, reference indices of L0 and L1, a motion vector difference, andinformation indicating whether or not the mode is a subblock-basedmotion vector predictor mode (a subblock-based motion vector predictorflag) as the coding information to the bit strings coding unit 108.Further, the prediction method determination unit 105 supplies thedetermined coding information to the coding information storage memory111. The prediction method determination unit 105 supplies a predictedpicture signal to the residual generation unit 106 and the decodingpicture signal superimposition unit 110.

The residual generation unit 106 generates a residual by subtracting thepredicted picture signal from the target picture signal and supplies theresidual to the orthogonal transform/quantization unit 107.

The orthogonal transform/quantization unit 107 performs an orthogonaltransform and quantization on the residual in accordance with thequantization parameter to generate an orthogonally transformed/quantizedresidual and supplies the generated residual to the bit strings codingunit 108 and the inverse quantization/inverse orthogonal transform unit109.

The bit strings coding unit 108 codes coding information according tothe prediction method determined by the prediction method determinationunit 105 for each coding block in addition to information of units ofsequences, pictures, slices, and coding blocks. Specifically, the bitstrings coding unit 108 codes the prediction mode PredMode for eachcoding block. When the prediction mode is inter prediction (MODE_INTER),the bit strings coding unit 108 codes coding information (interprediction information) such as a flag for discriminating whether or notthe mode is a merge mode, a subblock-based merge flag, a merge indexwhen the mode is the merge mode, an inter prediction mode when the modeis not the merge mode, a motion vector predictor index, informationabout a motion vector difference, and a subblock-based motion vectorpredictor flag in accordance with specified syntax (a bit strings syntaxrule) and generates first bit strings. When the prediction mode is intraprediction (MODE_INTRA), coding information (intra predictioninformation) such as the intra prediction mode is coded in accordancewith specified syntax (a bit strings syntax rule) and first bit stringsis generated. Also, the bit strings coding unit 108 entropy-codes theorthogonally transformed and quantized residual in accordance withspecified syntax to generate second bit strings. The bit strings codingunit 108 multiplexes the first bit strings and the second bit strings inaccordance with specified syntax and outputs a bitstream.

The inverse quantization/inverse orthogonal transform unit 109calculates the residual by performing inverse quantization and aninverse orthogonal transform on the orthogonally transformed/quantizedresidual supplied from the orthogonal transform/quantization unit 107and supplies the calculated residual to the decoding picture signalsuperimposition unit 110.

The decoding picture signal superimposition unit 110 superimposes thepredicted picture signal according to the determination of theprediction method determination unit 105 and the residual inverselyquantized and inversely orthogonally transformed by the inversequantization/inverse orthogonal transform unit 109 to generate a decodedpicture and stores the decoded picture in the decoded picture memory104. Also, the decoding picture signal superimposition unit 110 maystore the decoded picture in the decoded picture memory 104 afterperforming a filtering process of reducing distortion such as blockdistortion due to coding on the decoded picture.

The coding information storage memory 111 stores coding information suchas a prediction mode (inter prediction or intra prediction) determinedby the prediction method determination unit 105. In the case of theinter prediction, the coding information stored in the codinginformation storage memory 111 includes inter prediction informationsuch as a determined motion vector, reference indices of reference listsL0 and L1, and a history-based motion vector predictor candidate list.Also, in the case of the inter prediction merge mode, the codinginformation stored in the coding information storage memory 111 includesinter prediction information such as a merge index and informationindicating whether or not the mode is the subblock-based merge mode (asubblock-based merge flag) in addition to the above-describedinformation. Also, in the case of the motion vector predictor mode ofthe inter prediction, the coding information stored in the codinginformation storage memory 111 includes inter prediction informationsuch as an inter prediction mode, a motion vector predictor index, amotion vector difference, and information indicating whether or not themode is the subblock-based motion vector predictor mode (asubblock-based motion vector predictor flag) in addition to theabove-described information. In the case of the intra prediction, thecoding information stored in the coding information storage memory 111includes intra prediction information such as the determined intraprediction mode.

FIG. 2 is a block diagram showing a configuration of the picturedecoding device according to the embodiment of the present inventioncorresponding to the picture coding device of FIG. 1 . The picturedecoding device according to the embodiment includes a bit stringsdecoding unit 201, a block split unit 202, an inter prediction unit 203,an intra prediction unit 204, a coding information storage memory 205,an inverse quantization/inverse orthogonal transform unit 206, adecoding picture signal superimposition unit 207, and a decoded picturememory 208.

Because a decoding process of the picture decoding device of FIG. 2corresponds to a decoding process provided in the picture coding deviceof FIG. 1 , the components of the coding information storage memory 205,the inverse quantization/inverse orthogonal transform unit 206, thedecoding picture signal superimposition unit 207, and the decodedpicture memory 208 of FIG. 2 have functions corresponding to thecomponents of the coding information storage memory 111, the inversequantization/inverse orthogonal transform unit 109, the decoding picturesignal superimposition unit 110, and the decoded picture memory 104 ofthe picture coding device of FIG. 1 .

A bitstream supplied to the bit strings decoding unit 201 is separatedin accordance with a specified syntax rule. The bit strings decodingunit 201 decodes a separated first bit string, and obtains informationof units of sequences, pictures, slices, coding blocks and codinginformation of units of coding blocks. Specifically, the bit stringsdecoding unit 201 decodes a prediction mode PredMode for discriminatinginter prediction (MODE_INTER) or intra prediction (MODE_INTRA) in unitsof coding blocks. When the prediction mode is inter prediction(MODE_INTER), the bit strings decoding unit 201 decodes codinginformation (inter prediction information) about a flag fordiscriminating whether or not the mode is a merge mode, a merge indexwhen the mode is the merge mode, a subblock-based merge flag, an interprediction mode when the mode is a motion vector predictor mode, amotion vector predictor index, a motion vector difference, asubblock-based motion vector predictor flag, and the like in accordancewith specified syntax and supplies the coding information (the interprediction information) to the coding information storage memory 205 viathe inter prediction unit 203 and the block split unit 202. When theprediction mode is intra prediction (MODE_INTRA), coding information(intra prediction information) such as the intra prediction mode isdecoded in accordance with specified syntax and the coding information(the intra prediction information) is supplied to the coding informationstorage memory 205 via the inter prediction unit 203 or the intraprediction unit 204 and the block split unit 202. The bit stringsdecoding unit 201 decodes separated second bit strings to calculate anorthogonally transformed/quantized residual and supplies theorthogonally transformed/quantized residual to the inversequantization/inverse orthogonal transform unit 206.

When the prediction mode PredMode of the target coding block is themotion vector predictor mode in the inter prediction (MODE_INTER), theinter prediction unit 203 derives a plurality of motion vector predictorcandidates using coding information of the previously decoded picturesignal stored in the coding information storage memory 205 and registersthe plurality of derived motion vector predictor candidates in themotion vector predictor candidate list to be described below. The interprediction unit 203 selects a motion vector predictor according to themotion vector predictor index decoded and supplied by the bit stringsdecoding unit 201 from among the plurality of motion vector predictorcandidates registered in the motion vector predictor candidate list,calculates a motion vector from the motion vector difference decoded bythe bit strings decoding unit 201 and the selected motion vectorpredictor, and stores the calculated motion vector in the codinginformation storage memory 205 together with other coding information.The coding information of the coding block supplied/stored here is aprediction mode PredMode, flags predFlagL0[xP][yP] andpredFlagL1[xP][yP] indicating whether or not to use L0-prediction andL1-prediction, reference indices refIdxL0[xP][yP] and refIdxL1[xP][yP]of L0 and L1, motion vectors mvL0[xP][yP] and mvL1[xP][yP] of L0 and L1,and the like. Here, xP and yP are indices indicating a position of anupper left sample of the coding block within the picture. When theprediction mode PredMode is inter prediction (MODE_INTER) and the interprediction mode is L0-prediction (Pred_L0), the flag predFlagL0indicating whether or not to use L0-prediction is 1, and the flagpredFlagL1 indicating whether or not to use L1-prediction is 0. When theinter prediction mode is L1-prediction (Pred_L1), the flag predFlagL0indicating whether or not to use L0-prediction is 0 and the flagpredFlagL1 indicating whether or not to use L1-prediction is 1. When theinter prediction mode is bi-prediction (Pred_BI), both the flagpredFlagL0 indicating whether or not to use L0-prediction and the flagpredFlagL1 indicating whether or not to use L1-prediction are 1.Further, merging candidates are derived in the merge mode in which theprediction mode PredMode of the coding block of the target is interprediction (MODE_INTER). A plurality of merging candidates are derivedusing the coding information of the previously decoded coding blocksstored in the coding information storage memory 205 and are registeredin a merging candidate list to be described below, a merging candidatecorresponding to a merge index to be decoded and supplied by the bitstrings decoding unit 201 is selected from among the plurality ofmerging candidates registered in the merging candidate list, and interprediction information such as the flags predFlagL0[xP][yP] andpredFlagL1[xP][yP] indicating whether or not to use L0-prediction andL1-prediction of the selected merging candidate, the reference indicesrefIdxL0[xP][yP] and refIdxL1[xP][yP] of L0 and L1, and the motionvectors mvL0[xP][yP] and mvL1[xP][yP] of L0 and L1 is stored in thecoding information storage memory 205. Here, xP and yP are indicesindicating the position of the upper left sample of the coding block inthe picture. A detailed configuration and operation of the interprediction unit 203 will be described below.

The intra prediction unit 204 performs intra prediction when theprediction mode PredMode of the coding block of the target is intraprediction (MODE_INTRA). The coding information decoded by the bitstrings decoding unit 201 includes an intra prediction mode. The intraprediction unit 204 generates a predicted picture signal according tointra prediction from the decoded picture signal stored in the decodedpicture memory 208 in accordance with the intra prediction mode includedin the coding information decoded by the bit strings decoding unit 201and supplies the generated predicted picture signal to the decodingpicture signal superimposition unit 207. Because the intra predictionunit 204 corresponds to the intra prediction unit 103 of the picturecoding device 100, a process similar to that of the intra predictionunit 103 is performed.

The inverse quantization/inverse orthogonal transform unit 206 performsan inverse orthogonal transform and inverse quantization on theorthogonally transformed/quantized residual decoded by the bit stringsdecoding unit 201 and obtains the inversely orthogonallytransformed/inversely quantized residual.

The decoding picture signal superimposition unit 207 decodes a decodingpicture signal by superimposing a predicted picture signalinter-predicted by the inter prediction unit 203 or a predicted picturesignal intra-predicted by the intra prediction unit 204 and the residualinversely orthogonally transformed/inversely quantized by the inversequantization/inverse orthogonal transform unit 206 and stores thedecoded decoding picture signal in the decoded picture memory 208. Atthe time of storage in the decoded picture memory 208, the decodingpicture signal superimposition unit 207 may store a decoded picture inthe decoded picture memory 208 after a filtering process of reducingblock distortion or the like due to coding is performed on the decodedpicture.

Next, an operation of the block split unit 101 in the picture codingdevice 100 will be described. FIG. 3 is a flowchart showing an operationof dividing a picture into tree blocks and further dividing each treeblock. First, an input picture is divided into tree blocks having apredetermined size (step S1001). Each tree block is scanned in apredetermined order, i.e., raster scan order (step S1002), and theinside of the tree block of a target is divided (step S1003).

FIG. 7 is a flowchart showing a detailed operation of a split process ofstep S1003. First, it is determined whether or not a target block willbe divided into four parts (step S1101).

When it is determined that the target block will be divided into fourparts, the target block is divided into four parts (step S1102). Eachblock obtained by dividing the target block is scanned in a Z-scanorder, i.e., in the order of upper left, upper right, lower left, andlower right (step S1103). FIG. 5 shows an example of the Z-scan order,and reference numeral 601 of FIG. 6A shows an example in which thetarget block is divided into four parts. Numbers 0 to 3 of referencenumeral 601 of FIG. 6A indicate the order of processing. Then, the splitprocess of FIG. 7 is recursively executed for each block from thedivision in step S1101 (step S1104).

When it is determined that the target block will not be divided intofour parts, a binary-ternary split is performed (step S1105).

FIG. 8 is a flowchart showing the detailed operation of a binary-ternarysplit process of step S1105. First, it is determined whether or not atarget block will be divided into two or three parts, i.e., whether ornot either a binary or ternary split will be performed (step S1201).

When it is not determined that the target block will be divided into twoor three parts, i.e., when it is determined that the target block willnot be divided, the split ends (step S1211). That is, a recursive splitprocess is not further performed on blocks divided according to therecursive split process.

When it is determined that the target block will be divided into two orthree parts, it is further determined whether or not the target blockwill be divided into two parts (step S1202).

When it is determined that the target block will be divided into twoparts, it is determined whether or not the target block will be dividedinto upper and lower parts (in a vertical direction) (step S1203). Onthe basis of a determination result, the target block is divided intotwo parts that are upper and lower parts (in the vertical direction)(step S1204) or the target block is divided into two parts that are leftand right parts (in a horizontal direction) (step S1205). As a result ofstep S1204, the target block is divided into two parts that are upperand lower parts (in the vertical direction) as indicated by referencenumeral 602 in FIG. 6B. As a result of step S1205, the target block isdivided into two parts that are left and right parts (in the horizontaldirection) as indicated by reference numeral 604 of FIG. 6D.

When it is not determined that the target block will be divided into twoparts, i.e., when it is determined that the target block will be dividedinto three parts, in step S1202, it is determined whether or not thetarget block will be divided into upper, middle, and lower parts (in thevertical direction) (step S1206). On the basis of a determinationresult, the target block is divided into three parts that are upper,middle and lower parts (in the vertical direction) (step S1207) or thetarget block is divided into three parts that are left, middle, andright parts (in the horizontal direction) (step S1208). As a result ofstep S1207, the target block is divided into three parts that are upper,middle, and lower parts (in the vertical direction) as indicated byreference numeral 603 of FIG. 6C. As a result of step S1208, the targetblock is divided into three parts that are left, middle, and right parts(in the horizontal direction) as indicated by reference numeral 605 ofFIG. 6E.

After any one of steps S1204, S1205, S1207, and S1208 is executed, eachof blocks into which the target block is divided is scanned in orderfrom left to right and from top to bottom (step S1209). Numbers 0 to 2of reference numerals 602 to 605 of FIGS. 6B to 6E indicate the order ofprocessing. For each of the blocks into which the target block isdivided, a binary-ternary split process of FIG. 8 is recursivelyexecuted (step S1210).

The recursive block split described here may limit the necessity of asplit according to the number of splits or a size of the target block orthe like. Information that limits the necessity of a split may beimplemented by a configuration in which information is not delivered bymaking an agreement between the coding device and the decoding device inadvance or implemented by a configuration in which the coding devicedetermines information that limits the necessity of a split, records theinformation in a bit string, and delivers the information to thedecoding device.

When a certain block is divided, a block before the split is referred toas a parent block and each block after the split is referred to as achild block.

Next, an operation of the block split unit 202 in the picture decodingdevice 200 will be described. The block split unit 202 divides the treeblock according to a processing procedure similar to that of the blocksplit unit 101 of the picture coding device 100. However, there is adifference in that the block split unit 101 of the picture coding device100 applies an optimization technique such as estimation of an optimumshape based on picture recognition or distortion rate optimization todetermine an optimum block split shape, whereas the block split unit 202of the picture decoding device 200 determines a block split shape bydecoding the block split information recorded in the bit string.

Syntax (a bit strings syntax rule) related to a block split according tothe first embodiment is shown in FIG. 9 . coding_quadtree( ) representssyntax related to a quad split process on the block. multi_type_tree( )represents syntax related to a binary or ternary split process on ablock. qt_split is a flag indicating whether or not a block is dividedinto four parts. qt_split=1 when the block is divided into four partsand qt_split=0 when the block is not divided into four parts. When theblock is divided into four parts (qt_split=1), a quad split process isrecursively performed on blocks, each of which has been divided intofour parts (coding_quadtree(0), coding_quadtree(1), coding_quadtree(2),coding_quadtree(3), and arguments 0 to 3 correspond to numbers indicatedby reference numeral 601 of FIG. 6A). When the block is not divided intofour parts (qt_split=0), the subsequent split is determined according tomulti_type_tree( ). mtt_split is a flag indicating whether or not asplit is further performed. When a split is further performed(mtt_split=1), mtt_split_vertical which is a flag indicating whether theblock is divided vertically or horizontally and mtt_split_binary whichis a flag for determining whether a binary or ternary split is performedare transmitted. mtt_split_vertical=1 indicates a split in the verticaldirection and mtt_split_vertical=0 indicates a split in the horizontaldirection. mtt_split_binary=1 indicates a binary split andmtt_split_binary=0 indicates a ternary split. In the binary split(mtt_split_binary=1), a split process is recursively performed onblocks, each of which is divided into two parts (multi_type_tree(0),multi_type_tree(1), and arguments 0 to 1 correspond to numbers indicatedby reference numeral 602 or 604 in FIGS. 6B to 6D). In the case of theternary split (mtt_split_binary=0), a split process is recursivelyperformed on blocks, each of which is divided into three parts(multi_type_tree(0), multi_type_tree(1), multi_type_tree(2), andarguments 0 to 2 correspond to numbers indicated by reference numeral603 of FIG. 6B or numbers indicated by reference numeral 605 of FIG.6E). Until mtt_split=0 is reached, a hierarchical block split isperformed by recursively calling multi_type_tree.

<Inter Prediction>

An inter prediction method according to the embodiment is performed inthe inter prediction unit 102 of the picture coding device of FIG. 1 andthe inter prediction unit 203 of the picture decoding device of FIG. 2 .

The inter prediction method according to the embodiment will bedescribed with reference to the drawings. The inter prediction method isperformed in both coding and decoding processes in units of codingblocks.

<Description of Inter Prediction Unit 102 of Coding Side>

FIG. 16 is a diagram showing a detailed configuration of the interprediction unit 102 of the picture coding device in FIG. 1 . The normalmotion vector predictor mode derivation unit 301 derives a plurality ofnormal motion vector predictor candidates to select a motion vectorpredictor, and calculates a motion vector difference between theselected motion vector predictor and a detected motion vector. Adetected inter prediction mode, reference index, and motion vector andthe calculated motion vector difference become inter predictioninformation of the normal motion vector predictor mode. This interprediction information is supplied to the inter prediction modedetermination unit 305. A detailed configuration and a process of thenormal motion vector predictor mode derivation unit 301 will bedescribed below.

The normal merge mode derivation unit 302 derives a plurality of normalmerging candidates to select a normal merging candidate and obtainsinter prediction information of the normal merge mode. This interprediction information is supplied to the inter prediction modedetermination unit 305. A detailed configuration and a process of thenormal merge mode derivation unit 302 will be described below.

A subblock-based motion vector predictor mode derivation unit 303derives a plurality of subblock-based motion vector predictor candidatesto select a subblock-based motion vector predictor and calculates amotion vector difference between the selected subblock-based motionvector predictor and the detected motion vector. A detected interprediction mode, reference index, and motion vector and the calculatedmotion vector difference become the inter prediction information of thesubblock-based motion vector predictor mode. This inter predictioninformation is supplied to the inter prediction mode determination unit305.

The subblock-based merge mode derivation unit 304 derives a plurality ofsubblock-based merging candidates to select a subblock-based mergingcandidate, and obtains inter prediction information of thesubblock-based merge mode. This inter prediction information is suppliedto the inter prediction mode determination unit 305.

The inter prediction mode determination unit 305 determines interprediction information on the basis of the inter prediction informationsupplied from the normal motion vector predictor mode derivation unit301, the normal merge mode derivation unit 302, the subblock-basedmotion vector predictor mode derivation unit 303, and the subblock-basedmerge mode derivation unit 304. Inter prediction information accordingto the determination result is supplied from the inter prediction modedetermination unit 305 to the motion-compensated prediction unit 306.

The motion-compensated prediction unit 306 performs inter prediction onthe reference picture signal stored in the decoded picture memory 104 onthe basis of the determined inter prediction information. A detailedconfiguration and a process of the motion-compensated prediction unit306 will be described below.

<Description of Inter Prediction Unit 203 of Decoding Side>

FIG. 22 is a diagram showing a detailed configuration of the interprediction unit 203 of the picture decoding device of FIG. 2 .

A normal motion vector predictor mode derivation unit 401 derives aplurality of normal motion vector predictor candidates to select amotion vector predictor, calculates a sum of the selected motion vectorpredictor and the decoded motion vector difference, and sets thecalculated sum as a motion vector. A decoded inter prediction mode,reference index, and motion vector become inter prediction informationof the normal motion vector predictor mode. This inter predictioninformation is supplied to a motion-compensated prediction unit 406 viathe switch 408. A detailed configuration and a process of the normalmotion vector predictor mode derivation unit 401 will be describedbelow.

A normal merge mode derivation unit 402 derives a plurality of normalmerging candidates to select a normal merging candidate and obtainsinter prediction information of the normal merge mode. This interprediction information is supplied to the motion-compensated predictionunit 406 via the switch 408. A detailed configuration and a process ofthe normal merge mode derivation unit 402 will be described below.

A subblock-based motion vector predictor mode derivation unit 403derives a plurality of subblock-based motion vector predictor candidatesto select a subblock-based motion vector predictor, calculates a sum ofthe selected subblock-based motion vector predictor and the decodedmotion vector difference, and sets the calculated sum as a motionvector. A decoded inter prediction mode, reference index, and motionvector become the inter prediction information of the subblock-basedmotion vector predictor mode. This inter prediction information issupplied to the motion-compensated prediction unit 406 via the switch408.

A subblock-based merge mode derivation unit 404 derives a plurality ofsubblock-based merging candidates to select a subblock-based mergingcandidate and obtains inter prediction information of the subblock-basedmerge mode. This inter prediction information is supplied to themotion-compensated prediction unit 406 via the switch 408.

The motion-compensated prediction unit 406 performs inter prediction onthe reference picture signal stored in the decoded picture memory 208 onthe basis of the determined inter prediction information. A detailedconfiguration and a process of the motion-compensated prediction unit406 are similar to those of the motion-compensated prediction unit 306of the coding side.

<Normal Motion Vector Predictor Mode Derivation Unit (Normal AMVP)>

The normal motion vector predictor mode derivation unit 301 of FIG. 17includes a spatial motion vector predictor candidate derivation unit321, a temporal motion vector predictor candidate derivation unit 322, ahistory-based motion vector predictor candidate derivation unit 323, amotion vector predictor candidate replenishment unit 325, a normalmotion vector detection unit 326, a motion vector predictor candidateselection unit 327, and a motion vector subtraction unit 328.

The normal motion vector predictor mode derivation unit 401 of FIG. 23includes a spatial motion vector predictor candidate derivation unit421, a temporal motion vector predictor candidate derivation unit 422, ahistory-based motion vector predictor candidate derivation unit 423, amotion vector predictor candidate replenishment unit 425, a motionvector predictor candidate selection unit 426, and a motion vectoraddition unit 427.

Processing procedures of the normal motion vector predictor modederivation unit 301 of the coding side and the normal motion vectorpredictor mode derivation unit 401 of the decoding side will bedescribed using the flowcharts of FIGS. 19 and 25 , respectively. FIG.19 is a flowchart showing a normal motion vector predictor modederivation processing procedure of the normal motion vector predictormode derivation unit 301 of the coding side and FIG. 25 is a flowchartshowing a normal motion vector predictor mode derivation processingprocedure of the normal motion vector predictor mode derivation unit 401of the decoding side.

<Normal Motion Vector Predictor Mode Derivation Unit (Normal AMVP):Description of Coding Side>

The normal motion vector predictor mode derivation processing procedureof the coding side will be described with reference to FIG. 19 . In thedescription of the processing procedure of FIG. 19 , the term “normal”shown in FIG. 19 may be omitted.

First, the normal motion vector detection unit 326 detects a normalmotion vector for each inter prediction mode and each reference index(step S100 of FIG. 19 ).

Subsequently, in the spatial motion vector predictor candidatederivation unit 321, the temporal motion vector predictor candidatederivation unit 322, the history-based motion vector predictor candidatederivation unit 323, the motion vector predictor candidate replenishmentunit 325, the motion vector predictor candidate selection unit 327, andthe motion vector subtraction unit 328, a motion vector difference of amotion vector used for inter prediction of the normal motion vectorpredictor mode is calculated for each of L0 and L1 (steps S101 to S106of FIG. 19 ). Specifically, when the prediction mode PredMode of thetarget block is inter prediction (MODE_INTER) and the inter predictionmode is L0-prediction (Pred_L0), the motion vector predictor candidatelist mvpListL0 of L0 is calculated to select the motion vector predictormvpL0 and the motion vector difference mvdL0 of the motion vector mvL0of L0 is calculated. When the inter prediction mode of the target blockis L1-prediction (Pred_L1), the motion vector predictor candidate listmvpListL1 of L1 is calculated to select the motion vector predictormvpL1 and the motion vector difference mvdL1 of the motion vector mvL1of L1 is calculated. When the inter prediction mode of the target blockis bi-prediction (Pred_BI), both L0-prediction and L1-prediction areperformed, the motion vector predictor candidate list mvpListL0 of L0 iscalculated to select a motion vector predictor mvpL0 of L0, the motionvector difference mvdL0 of a motion vector mvL0 of L0 is calculated, themotion vector predictor candidate list mvpListL1 of L1 is calculated toselect a motion vector predictor mvpL1 of L1, and a motion vectordifference mvdL1 of a motion vector mvL1 of L1 is calculated.

Although a motion vector difference calculation process is performed foreach of L0 and L1, the motion vector difference calculation processbecomes a process common to both L0 and L1. Therefore, in the followingdescription, L0 and L1 are represented as common LX. X of LX is 0 in theprocess of calculating the motion vector difference of L0 and X of LX is1 in the process of calculating the motion vector difference of L1.Also, when information of another list instead of LX is referred toduring the process of calculating the motion vector difference of LX,the other list is represented as LY.

When the motion vector mvLX of LX is used (step S102 of FIG. 19 : YES),the motion vector predictor candidates of LX are calculated to constructthe motion vector predictor candidate list mvpListLX of LX (step S103 ofFIG. 19 ). In the spatial motion vector predictor candidate derivationunit 321, the temporal motion vector predictor candidate derivation unit322, the history-based motion vector predictor candidate derivation unit323, and the motion vector predictor candidate replenishment unit 325 ofthe normal motion vector predictor mode derivation unit 301, a pluralityof motion vector predictor candidates are derived to construct themotion vector predictor candidate list mvpListLX. The detailedprocessing procedure of step S103 of FIG. 19 will be described belowusing the flowchart of FIG. 20 .

Subsequently, the motion vector predictor candidate selection unit 327selects a motion vector predictor mvpLX of LX from the motion vectorpredictor candidate list mvpListLX of LX (step S104 of FIG. 19 ). Here,one element (an i^(th) element when counted from a 0^(th) element) inthe motion vector predictor candidate list mvpListLX is represented asmvpListLX[i]. Each motion vector difference that is a difference betweenthe motion vector mvLX and each motion vector predictor candidatemvpListLX[i] stored in the motion vector predictor candidate listmvpListLX is calculated. A code amount when the motion vectordifferences are coded is calculated for each element (motion vectorpredictor candidate) of the motion vector predictor candidate listmvpListLX. Then, a motion vector predictor candidate mvpListLX[i] thatminimizes the code amount for each motion vector predictor candidateamong the elements registered in the motion vector predictor candidatelist mvpListLX is selected as the motion vector predictor mvpLX and itsindex i is acquired. When there are a plurality of motion vectorpredictor candidates having the smallest generated code amount in themotion vector predictor candidate list mvpListLX, a motion vectorpredictor candidate mvpListLX[i] represented by a smaller number in theindex i in the motion vector predictor candidate list mvpListLX isselected as an optimum motion vector predictor mvpLX and its index i isacquired.

Subsequently, the motion vector subtraction unit 328 subtracts theselected motion vector predictor mvpLX of LX from the motion vector mvLXof LX and calculates a motion vector difference mvdLX of LX asmvdLX=mvLX-mvpLX (step S105 of FIG. 19 ).

<Normal Motion Vector Predictor Mode Derivation Unit (Normal AMVP):Description of Decoding Side>

Next, the normal motion vector predictor mode processing procedure ofthe decoding side will be described with reference to FIG. 25 . On thedecoding side, in the spatial motion vector predictor candidatederivation unit 421, the temporal motion vector predictor candidatederivation unit 422, the history-based motion vector predictor candidatederivation unit 423, and the motion vector predictor candidatereplenishment unit 425, a motion vector for use in inter prediction ofthe normal motion vector predictor mode is calculated for each of L0 andL1 (steps S201 to S206 of FIG. 25 ). Specifically, when the predictionmode PredMode of the target block is inter prediction (MODE_INTER) andthe inter prediction mode of the target block is L0-prediction(Pred_L0), the motion vector predictor candidate list mvpListL0 of L0 iscalculated to select the motion vector predictor mvpL0 and a motionvector mvL0 of L0 is calculated. When the inter prediction mode of thetarget block is L1-prediction (Pred_L1), the motion vector predictorcandidate list mvpListL1 of L1 is calculated to select the motion vectorpredictor mvpL1 and the motion vector mvL1 of L1 is calculated. When theinter prediction mode of the target block is bi-prediction (Pred_BI),both L0-prediction and L1-prediction are performed, the motion vectorpredictor candidate list mvpListL0 of L0 is calculated to select amotion vector predictor mvpL0 of L0, a motion vector mvL0 of L0 iscalculated, the motion vector predictor candidate list mvpListL1 of L1is calculated to select a motion vector predictor mvpL1 of L1, and eachmotion vector mvL1 of L1 is calculated.

Although a motion vector calculation process is performed for each of L0and L1 on the decoding side as on the coding side, the motion vectorcalculation process becomes a process common to both L0 and L1.Therefore, in the following description, L0 and L1 are represented ascommon LX. LX represents an inter prediction mode for use in the interprediction of a target coding block. X is 0 in the process ofcalculating the motion vector of L0 and X is 1 in the process ofcalculating the motion vector of L1. Also, when information of anotherreference list instead of a reference list identical to that of LX of acalculation target is referred to during the process of calculating themotion vector of LX, the other reference list is represented as LY.

When the motion vector mvLX of LX is used (step S202 of FIG. 25 : YES),the motion vector predictor candidates of LX are calculated to constructthe motion vector predictor candidate list mvpListLX of LX (step S203 ofFIG. 25 ). In the spatial motion vector predictor candidate derivationunit 421, the temporal motion vector predictor candidate derivation unit422, the history-based motion vector predictor candidate derivation unit423, and the motion vector predictor candidate replenishment unit 425 ofthe normal motion vector predictor mode derivation unit 401, a pluralityof motion vector predictor candidates are calculated to construct amotion vector predictor candidate list mvpListLX. A detailed processingprocedure of step S203 of FIG. 25 will be described below using theflowchart of FIG. 20 .

Subsequently, the motion vector predictor candidate mvpListLX[mvpIdxLX]corresponding to the index mvpIdxLX of the motion vector predictordecoded and supplied by the bit strings decoding unit 201 from themotion vector predictor candidate list mvpListLX is extracted as aselected motion vector predictor mvpLX in the motion vector predictorcandidate selection unit 426 (step S204 of FIG. 25 ).

Subsequently, the motion vector addition unit 427 sums the motion vectordifference mvdLX of LX that is decoded and supplied by the bit stringsdecoding unit 201 and the motion vector predictor mvpLX of LX andcalculates the motion vector mvLX of LX as mvLX=mvpLX+mvdLX (step S205of FIG. 25 ).

<Normal Motion Vector Predictor Mode Derivation Unit (Normal AMVP):Motion Vector Prediction Method>

FIG. 20 is a flowchart showing a processing procedure of a normal motionvector predictor mode derivation process having a function common to thenormal motion vector predictor mode derivation unit 301 of the picturecoding device and the normal motion vector predictor mode derivationunit 401 of the picture decoding device according to the embodiment ofthe present invention.

The normal motion vector predictor mode derivation unit 301 and thenormal motion vector predictor mode derivation unit 401 include a motionvector predictor candidate list mvpListLX. The motion vector predictorcandidate list mvpListLX has a list structure and is provided with astorage area where a motion vector predictor index indicating thelocation inside the motion vector predictor candidate list and a motionvector predictor candidate corresponding to the index are stored aselements. The number of the motion vector predictor index starts from 0and motion vector predictor candidates are stored in the storage area ofthe motion vector predictor candidate list mvpListLX. In the presentembodiment, it is assumed that at least two motion vector predictorcandidates (inter prediction information) can be registered in themotion vector predictor candidate list mvpListLX. Furthermore, avariable numCurrMvpCand indicating the number of motion vector predictorcandidates registered in the motion vector predictor candidate listmvpListLX is set to 0.

The spatial motion vector predictor candidate derivation units 321 and421 derive motion vector predictor candidates from neighboring blocks onthe left side. In this process, a motion vector predictor mvLXA isderived with reference to the inter prediction information of theneighboring block on the left side (A0 or A1 of FIG. 11 ), i.e., a flagindicating whether or not a motion vector predictor candidate can beused, a motion vector, a reference index, and the like, and the derivedmvLXA is added to the motion vector predictor candidate list mvpListLX(step S301 of FIG. 20 ). Also, X is 0 at the time of L0-prediction and Xis 1 at the time of L1-prediction (the same is true hereinafter).Subsequently, the spatial motion vector predictor candidate derivationunits 321 and 421 derive a motion vector predictor candidate from aneighboring block on the upper side. In this process, the motion vectorpredictor mvLXB is derived with reference to inter predictioninformation of a neighboring block on the upper side (B0, B1, or B2 ofFIG. 11 ), i.e., a flag indicating whether or not a motion vectorpredictor candidate can be used, a motion vector, a reference index, andthe like, and mvLXB is added to the motion vector predictor candidatelist mvpListLX if the derived mvLXA is not equal to the derived mvLXB(step S302 of FIG. 20 ). The processing of steps S301 and S302 of FIG.20 is common except that positions of neighboring blocks to be referredto and the number of neighboring blocks to be referred to are different,and a flag available FlagLXN indicating whether or not a motion vectorpredictor candidate of the coding block can be used, a motion vectormvLXN, and a reference index refIdxN (N represents A or B and the sameis true hereinafter) are derived.

Subsequently, the temporal motion vector predictor candidate derivationunits 322 and 422 derive motion vector predictor candidates from blocksin a picture whose time is different from that of the current targetpicture. In this process, a flag available FlagLXCol indicating whetheror not a motion vector predictor candidate of a coding block of apicture of different time can be used, a motion vector mvLXCol, areference index refIdxCol, and a reference list listCol are derived, andmvLXCol is added to the motion vector predictor candidate list mvpListLX(step S303 of FIG. 20 ).

Also, it is assumed that the processes of the temporal motion vectorpredictor candidate derivation units 322 and 422 can be omitted in unitsof sequences (SPS), pictures (PPS), or slices.

Subsequently, the history-based motion vector predictor candidatederivation units 323 and 423 add the history-based motion vectorpredictor candidates registered in the history-based motion vectorpredictor candidate list HmvpCandList to the motion vector predictorcandidate list mvpListLX (step S304 of FIG. 20 ). Details of theregistration processing procedure of step S304 will be described belowusing the flowchart of FIG. 29 .

Subsequently, the motion vector predictor candidate replenishment units325 and 425 add motion vector predictor candidates having apredetermined value such as (0, 0) until the motion vector predictorcandidate list mvpListLX is satisfied (S305 of FIG. 20 ).

<Normal Merge Mode Derivation Unit (Normal Merge)>

The normal merge mode derivation unit 302 of FIG. 18 includes a spatialmerging candidate derivation unit 341, a temporal merging candidatederivation unit 342, an average merging candidate derivation unit 344, ahistory-based merging candidate derivation unit 345, a merging candidatereplenishment unit 346, and a merging candidate selection unit 347.

The normal merge mode derivation unit 402 of FIG. 24 includes a spatialmerging candidate derivation unit 441, a temporal merging candidatederivation unit 442, an average merging candidate derivation unit 444, ahistory-based merging candidate derivation unit 445, a merging candidatereplenishment unit 446, and a merging candidate selection unit 447.

FIG. 21 is an explanatory flowchart showing a procedure of a normalmerge mode derivation process having a function common to the normalmerge mode derivation unit 302 of the picture coding device and thenormal merge mode derivation unit 402 of the picture decoding deviceaccording to the embodiment of the present invention.

Hereafter, various processes will be described step by step. Although acase in which a type of slice slice_type is a B slice will be describedunless otherwise specified in the following description, the presentinvention can also be applied to the case of a P slice. However, whenthe type of slice slice_type is a P slice, because only theL0-prediction (Pred_L0) is provided as the inter prediction mode andL1-prediction (Pred_L1) and bi-prediction (Pred_BI) are absent, aprocess related to L1 can be omitted.

The normal merge mode derivation unit 302 and the normal merge modederivation unit 402 have a merging candidate list mergeCandList. Themerging candidate list mergeCandList has a list structure and isprovided with a merge index indicating the location within the mergingcandidate list and a storage area where merging candidates correspondingto the index are stored as elements. The number of the merge indexstarts from 0 and merging candidates are stored in the storage area ofthe merging candidate list mergeCandList. In the subsequent process, themerging candidate of the merge index i registered in the mergingcandidate list mergeCandList is represented by mergeCandList[i]. In thepresent embodiment, it is assumed that at least six merging candidates(inter prediction information) can be registered in the mergingcandidate list mergeCandList. Further, a variable numCurrMergeCandindicating the number of merging candidates registered in the mergingcandidate list mergeCandList is set to 0.

In the spatial merging candidate derivation unit 341 and the spatialmerging candidate derivation unit 441, spatial merging candidates A andB from the neighboring blocks on the left side and the upper side of thetarget block are derived from the coding information stored in thecoding information storage memory 111 of the picture coding device orthe coding information storage memory 205 of the picture decoding deviceand the derived spatial merging candidates are registered in the mergingcandidate list mergeCandList (step S401 of FIG. 21 ). Here, N indicatingeither the spatial merging candidate A or B or the temporal mergingcandidate Col is defined. A flag available FlagN indicating whether ornot the inter prediction information of block N can be used as a spatialmerging candidate, a reference index refIdxL0N of L0 and a referenceindex refIdxL1N of L1 of spatial merging candidate N, an L0-predictionflag predFlagL0N indicating whether or not L0-prediction is performed,an L1-prediction flag predFlagL1N indicating whether or notL1-prediction is performed, a motion vector mvL0N of L0, and a motionvector mvL1N of L1 are derived. However, because the merging candidateis derived without referring to the inter prediction information of theblock included in the coding block which is a target in the presentembodiment, no spatial merging candidate using the inter predictioninformation of the block included in the target coding block is derived.

Subsequently, the temporal merging candidate derivation unit 342 and thetemporal merging candidate derivation unit 442 derive temporal mergingcandidates from pictures of different times and register the derivedtemporal merging candidates in the merging candidate list mergeCandList(step S402 of FIG. 21 ). A flag availableFlagCol indicating whether ornot the temporal merging candidate can be used, an L0-prediction flagpredFlagL0Col indicating whether or not L0-prediction of the temporalmerging candidate is performed, an L1-prediction flag predFlagL1Colindicating whether or not L1-prediction is performed, a motion vectormvL0Col of L0, and a motion vector mvL1Col of L1 are derived.

Also, it is assumed that the processes of the temporal merging candidatederivation unit 342 and the temporal merging candidate derivation unit442 can be omitted in units of sequences (SPS), pictures (PPS), orslices.

Subsequently, the history-based merging candidate derivation unit 345and the history-based merging candidate derivation unit 445 registerhistory-based motion vector predictor candidates registered in thehistory-based motion vector predictor candidate list HmvpCandList in themerging candidate list mergeCandList (step S403 of FIG. 21 ).

Also, when the number of merging candidates numCurrMergeCand registeredwithin the merging candidate list mergeCandList is smaller than themaximum number of merging candidates MaxNumMergeCand, the maximum numberof merging candidates MaxNumMergeCand is set as an upper limit of thenumber of merging candidates numCurrMergeCand registered within themerging candidate list mergeCandList and history-based mergingcandidates are derived and registered in the merging candidate listmergeCandList.

Subsequently, the average merging candidate derivation unit 344 and theaverage merging candidate derivation unit 444 derive an average mergingcandidate from the merging candidate list mergeCandList and adds thederived average merging candidate to the merging candidate listmergeCandList (step S404 of FIG. 21 ).

Also, when the number of merging candidates numCurrMergeCand registeredwithin the merging candidate list mergeCandList is smaller than themaximum number of merging candidates MaxNumMergeCand, the maximum numberof merging candidates MaxNumMergeCand is set as an upper limit of thenumber of merging candidates numCurrMergeCand registered within themerging candidate list mergeCandList and average merging candidates arederived and registered in the merging candidate list mergeCandList.

Here, the average merging candidate is a new merging candidate having amotion vector obtained by averaging motion vectors of a first mergingcandidate and a second merging candidate registered in the mergingcandidate list mergeCandList for each of the L0-prediction and theL1-prediction.

Subsequently, in the merging candidate replenishment unit 346 and themerging candidate replenishment unit 446, when the number of mergingcandidates numCurrMergeCand registered within the merging candidate listmergeCandList is smaller than the maximum number of merging candidatesMaxNumMergeCand, the maximum number of merging candidatesMaxNumMergeCand is set as an upper limit of the number of mergingcandidates numCurrMergeCand registered within the merging candidate listmergeCandList and an additional merging candidate is derived andregistered in the merging candidate list mergeCandList (step S405 ofFIG. 21 ). In the P slice, a merging candidate for which a motion vectorhas a value of (0, 0) and the prediction mode is L0-prediction (Pred_L0)is added using the maximum number of merging candidates MaxNumMergeCandas the upper limit. In the B slice, a merging candidate for which amotion vector has a value of (0, 0) and the prediction mode isbi-prediction (Pred_BI) is added. A reference index when the mergingcandidate is added is different from the previously added referenceindex.

Subsequently, the merging candidate selection unit 347 and the mergingcandidate selection unit 447 select merging candidates from the mergingcandidates registered within the merging candidate list mergeCandList.The merging candidate selection unit 347 of the coding side selects amerging candidate by calculating a code amount and a distortion amount,and supplies a merge index indicating the selected merging candidate andinter prediction information of the merging candidate to themotion-compensated prediction unit 306 via the inter prediction modedetermination unit 305. On the other hand, the merging candidateselection unit 447 of the decoding side selects a merging candidate onthe basis of a decoded merge index and supplies the selected mergingcandidate to the motion-compensated prediction unit 406.

<Update of History-Based Motion Vector Predictor Candidate List>

Next, an initialization method and an update method of the history-basedmotion vector predictor candidate list HmvpCandList provided in thecoding information storage memory 111 of the coding side and the codinginformation storage memory 205 of the decoding side will be described indetail. FIG. 26 is an explanatory flowchart showing a processingprocedure of initializing/updating a history-based motion vectorpredictor candidate list.

In the present embodiment, it is assumed that the history-based motionvector predictor candidate list HmvpCandList is updated in the codinginformation storage memory 111 and the coding information storage memory205. A history-based motion vector predictor candidate list update unitmay be installed in the inter prediction unit 102 and the interprediction unit 203 to update the history-based motion vector predictorcandidate list HmvpCandList.

The history-based motion vector predictor candidate list HmvpCandList isinitially set at the beginning of the slice, the history-based motionvector predictor candidate list HmvpCandList is updated when the normalmotion vector predictor mode or the normal merge mode has been selectedby the prediction method determination unit 105 on the coding side, andthe history-based motion vector predictor candidate list HmvpCandList isupdated when the prediction information decoded by the bit stringsdecoding unit 201 is about the normal motion vector predictor mode orthe normal merge mode on the decoding side.

The inter prediction information used when inter prediction is performedin the normal motion vector predictor mode or the normal merge mode isregistered as an inter prediction information candidate hMvpCand in thehistory-based motion vector predictor candidate list HmvpCandList. Theinter prediction information candidate hMvpCand includes a referenceindex refIdxL0 of L0, a reference index refIdxL1 of L1, an L0-predictionflag predFlagL0 indicating whether or not L0-prediction is performed, anL1-prediction flag predFlagL1 indicating whether or not L1-prediction isperformed, a motion vector mvL0 of L0, and a motion vector mvL1 of L1.

When there is inter prediction information having the same value as aninter prediction information candidate hMvpCand among elements (i.e.,inter prediction information) registered in the history-based motionvector predictor candidate list HmvpCandList provided in the codinginformation storage memory 111 of the coding side and the codinginformation storage memory 205 of the decoding side, the element isremoved from the history-based motion vector predictor candidate listHmvpCandList. On the other hand, when there is no inter predictioninformation having the same value as an inter prediction informationcandidate hMvpCand, the element at the beginning of the history-basedmotion vector predictor candidate list HmvpCandList is removed and theinter prediction information candidate hMvpCand is added to the end ofthe history-based motion vector predictor candidate list HmvpCandList.

The number of elements of the history-based motion vector predictorcandidate list HmvpCandList provided in the coding information storagememory 111 of the coding side and the coding information storage memory205 of the decoding side according to the present invention is assumedto be six.

First, the history-based motion vector predictor candidate listHmvpCandList is initialized in units of slices. History-based motionvector predictor candidates are added to all elements of thehistory-based motion vector predictor candidate list HmvpCandList at thebeginning of the slice and a value of the number of history-based motionvector predictor candidates NumHmvpCand registered in the history-basedmotion vector predictor candidate list HmvpCandList is set to 6 (stepS2101 of FIG. 26 ).

Here, the initialization of the history-based motion vector predictorcandidate list HmvpCandList is performed in units of slices (a firstcoding block of a slice), but may be performed in units of pictures,tiles, or tree block rows.

FIG. 38 is a table showing an example of history-based motion vectorpredictor candidates added according to initialization of thehistory-based motion vector predictor candidate list HmvpCandList. Anexample when a type of slice is a B slice and the number of referencepictures is 4 is shown. A history-based motion vector predictor index isa value from (the number of history-based motion vector predictorcandidates NumHmvpCand−1) to 0 and the history-based motion vectorpredictor candidate list is filled with history-based candidates byadding inter prediction information with a motion vector value of (0, 0)as a history-based motion vector predictor candidate to the motionvector predictor candidate list HmvpCandList in accordance with a typeof slice. At this time, the history-based motion vector predictor indexstarts from (the number of history-based motion vector predictorcandidates NumHmvpCand−1) and a value incremented by 1 from 0 to (thenumber of reference pictures numRefIdx−1) is set as the reference indexrefIdxLX (X is 0 or 1). Subsequently, overlapping between thehistory-based motion vector predictor candidates is allowed and thevalue of 0 is set in refIdxLX. Invalid history-based motion vectorpredictor candidates are eliminated by setting all values for the numberof history-based motion vector predictor candidates NumHmvpCand andsetting the value of the number of history-based motion vector predictorcandidates NumHmvpCand to a fixed value. In this manner, it is possibleto improve coding efficiency by generally allocating a small value to areference index of high selectivity from a candidate of a history-basedmotion vector predictor index of a highest probability having a largestvalue to be added to the motion vector predictor candidate list or themerging candidate list.

Also, because the number of the history-based motion vector predictorcandidates can be treated as a fixed value by filling the history-basedmotion vector predictor candidate list with the history-based motionvector predictor candidates in units of slices, for example, thehistory-based motion vector predictor candidate derivation process orthe history-based merging candidate derivation process can besimplified.

Here, although the value of the motion vector is generally set to (0, 0)at a high selection probability, it is only necessary for the value ofthe motion vector to be a predetermined value. For example, the codingefficiency of the motion vector difference may be improved by settingvalues such as (4, 4), (0, 32), (−128, 0) or the coding efficiency ofthe motion vector difference may be improved by setting a plurality ofpredetermined values.

Also, although it is assumed that the history-based motion vectorpredictor index starts from (the number of history-based motion vectorpredictor candidates NumHmvpCand−1) and a value incremented by 1 from 0to (the number of reference pictures numRefIdx−1) is set as thereference index refIdxLX (X is 0 or 1), the history-based motion vectorpredictor index may start from 0.

FIG. 39 is a table showing another example of history-based motionvector predictor candidates added according to the initialization of thehistory-based motion vector predictor candidate list HmvpCandList. Anexample when the type of slice is a B slice and the number of referencepictures is 2 is shown. In this example, the history-based motion vectorpredictor candidate list is filled by adding inter predictioninformation with a different reference index or a different motionvector value as a history-based motion vector predictor candidate sothat there is no overlapping between history-based motion vectorpredictor candidates in elements of the history-based motion vectorpredictor candidate list HmvpCandList. At this time, the history-basedmotion vector predictor index starts from (the number of history-basedmotion vector predictor candidates NumHmvpCand−1) and a valueincremented by 1 from 0 to (the number of reference picturesnumRefIdx−1) is set as the reference index refIdxLX (X is 0 or 1).Subsequently, motion vectors with different values are added ashistory-based motion vector predictor candidates when refIdxLX is 0.Invalid history-based motion vector predictor candidates are eliminatedby setting all values for the number of history-based motion vectorpredictor candidates NumHmvpCand and setting the value of the number ofhistory-based motion vector predictor candidates NumHmvpCand to a fixedvalue.

In this manner, it is possible to fill the history-based motion vectorpredictor candidate list with non-overlapping history-based motionvector predictor candidates in units of slices and it is furthermorepossible to omit a process of the merging candidate replenishment unit346 to be executed in units of coding blocks after the history-basedmerging candidate derivation unit 345 in the normal merge modederivation unit 302 to be described below and reduce an amount ofprocessing.

Here, although the value of the motion vector is set to a small valuesuch as (0, 0) or (1, 0), the value of the motion vector may beincreased as long as the history-based motion vector predictorcandidates do not overlap.

Also, although it is assumed that the history-based motion vectorpredictor index starts from (the number of history-based motion vectorpredictor candidates NumHmvpCand−1) and a value incremented by 1 from 0to (the number of reference pictures numRefIdx−1) is set as thereference index refIdxLX (X is 0 or 1), the history-based motion vectorpredictor index may start from 0.

FIG. 40 is a table showing another example of history-based motionvector predictor candidates added according to initialization of thehistory-based motion vector predictor candidate list HmvpCandList.

An example when a type of slice is a B slice is shown. In this example,the history-based motion vector predictor candidate list is filled byadding inter prediction information having different motion vectorvalues as history-based motion vector predictor candidates when thereference index is 0 so that there is no overlapping betweenhistory-based motion vector predictor candidates in elements of thehistory-based motion vector predictor candidate list HmvpCandList. Atthis time, the history-based motion vector predictor index starts from(the number of history-based motion vector predictor candidatesNumHmvpCand−1) and 0 is set as the reference index refIdxLX (X is 0 or1). Invalid history-based motion vector predictor candidates areeliminated by setting all values for the number of history-based motionvector predictor candidates NumHmvpCand and setting the value of thenumber of history-based motion vector predictor candidates NumHmvpCandto a fixed value.

In this manner, a process can be further simplified because it ispossible to perform the initialization without considering the number ofreference pictures by setting the reference index to 0.

Here, although it is assumed that the value of the motion vector is amultiple of 2, other values may be used as long as there is nooverlapping between the history-based motion vector predictor candidateswhen the reference index is 0.

Also, although it is assumed that the history-based motion vectorpredictor index starts from (the number of history-based motion vectorpredictor candidates NumHmvpCand−1) and a value incremented by 1 from 0to (the number of reference pictures numRefIdx−1) is set as thereference index refIdxLX (X is 0 or 1), the history-based motion vectorpredictor index may start from 0.

Subsequently, the following process of updating the history-based motionvector predictor candidate list HmvpCandList is iteratively performedfor each coding block within the slice (steps S2102 to S2111 of FIG. 26).

First, initial setting is performed in units of coding blocks. A flagidenticalCandExist indicating whether or not there is an identicalcandidate is set to a value of FALSE and a removal target indexremoveIdx is set to 0 (step S2103 of FIG. 26 ).

Also, the initialization of the history-based motion vector predictorcandidate list HmvpCandList is performed in units of slices (a firstcoding block of a slice), but may be performed in units of pictures,tiles, or tree block rows.

It is determined whether or not there is an inter prediction informationcandidate hMvpCand of the registration target (step S2104 of FIG. 26 ).When the prediction method determination unit 105 of the coding sidedetermines that the mode is the normal motion vector predictor mode orthe normal merge mode or when the bit strings decoding unit 201 of thedecoding side decodes the mode as the normal motion vector predictormode or the normal merge mode, its inter prediction information is setas an inter prediction information candidate hMvpCand of theregistration target. When the prediction method determination unit 105of the coding side determines that the mode is the intra-predictionmode, the subblock-based motion vector predictor mode, or thesubblock-based merge mode or when the bit strings decoding unit 201 ofthe decoding side decodes the mode as the intra-prediction mode, thesubblock-based motion vector predictor mode, or the subblock-based mergemode, a process of updating the history-based motion vector predictorcandidate list HmvpCandList is not performed and the inter predictioninformation candidate hMvpCand of the registration target does notexist. When there is no inter prediction information candidate hMvpCandof the registration target, steps S2105 to S2106 are skipped (step S2104of FIG. 26 : NO). When there is an inter prediction informationcandidate hMvpCand of the registration target, the processing from stepS2105 is performed (step S2104 of FIG. 26 : YES).

Subsequently, it is determined whether or not there is an element (interprediction information) having the same value as the inter predictioninformation candidate hMvpCand of the registration target, i.e., anidentical element, among elements of the history-based motion vectorpredictor candidate list HmvpCandList (step S2105 of FIG. 26 ). FIG. 27is a flowchart of an identical element checking processing procedure.When a value of the number of history-based motion vector predictorcandidates NumHmvpCand is 0 (step S2121 of FIG. 27 : NO), thehistory-based motion vector predictor candidate list HmvpCandList isempty and there is no identical candidate, so that steps S2122 to S2125of FIG. 27 are skipped and the present identical element checkingprocessing procedure is completed. When the value of the number ofhistory-based motion vector predictor candidates NumHmvpCand is greaterthan 0 (YES in step S2121 of FIG. 27 ), the processing of step S2123 isiterated until the history-based motion vector predictor index hMvpIdxchanges from 0 to NumHmvpCand−1 (steps S2122 to S2125 of FIG. 27 ).First, a comparison is made regarding whether or not an hMvpTdxthelement HmvpCandList[hMvpIdx] when counted from a₀th element of thehistory-based motion vector predictor candidate list is identical to theinter prediction information candidate hMvpCand (step S2123 of FIG. 27). When they are the same (step S2123 of FIG. 27 : YES), a flagidenticalCandExist indicating whether or not there is an identicalcandidate is set to a value of TRUE and a removal target index removeIdxindicating a position of an element of a removal target is set to acurrent value of the history-based motion vector predictor indexhMvpIdx, and the present identical element checking process ends. Whenthey are not the same (step S2123 of FIG. 27 : NO), hMvpIdx isincremented by 1. If the history-based motion vector predictor indexhMvpIdx is less than or equal to NumHmvpCand−1, the processing from stepS2123 is performed.

Here, it is possible to omit step S2121 of FIG. 27 by filling thehistory-based motion vector predictor candidate list with thehistory-based motion vector predictor candidates.

Referring to the flowchart of FIG. 26 again, a process of shifting andadding an element of the history-based motion vector predictor candidatelist HmvpCandList is performed (step S2106 of FIG. 26 ). FIG. 28 is aflowchart of a processing procedure of shifting/adding an element of thehistory-based motion vector predictor candidate list HmvpCandList ofstep S2106 of FIG. 26 . First, it is determined whether or not to add anew element after removing an element stored in the history-based motionvector predictor candidate list HmvpCandList or to add a new elementwithout removing the element. Specifically, a comparison is maderegarding whether or not the flag identicalCandExist indicating whetheror not an identical candidate exists is TRUE or NumHmvpCand is six (stepS2141 of FIG. 28 ). When either the condition that the flagidenticalCandExist indicating whether or not an identical candidateexists is TRUE or the condition that the current number of candidatesNumHmvpCand is six is satisfied (step S2141 of FIG. 28 : YES), a newelement is added after removing the element stored in the history-basedmotion vector predictor candidate list HmvpCandList. The initial valueof index i is set to a value of removeIdx+1. The element shift processof step S2143 is iterated from this initial value to NumHmvpCand (stepsS2142 to S2144 of FIG. 28 ). By copying the element of HmvpCandList[i]to HmvpCandList[i−1], the element is shifted forward (step S2143 of FIG.28 ) and i is incremented by 1 (steps S2142 to S2144 of FIG. 28 ).Subsequently, the inter prediction information candidate hMvpCand isadded to a (NumHmvpCand−1)^(th) element HmvpCandList[NumHmvpCand−1] whencounted from a 0^(th) element that corresponds to the end of thehistory-based motion vector predictor candidate list (step S2145 of FIG.28 ) and the present process of shifting/adding an element of thehistory-based motion vector predictor candidate list HmvpCandList ends.On the other hand, when neither the condition that the flagidenticalCandExist indicating whether or not an identical candidateexists is TRUE nor the condition that the current number of candidatesNumHmvpCand is six is satisfied (step S2141 of FIG. 28 : NO), the interprediction information candidate hMvpCand is added to the end of thehistory-based motion vector predictor candidate list without removing anelement stored in the history-based motion vector predictor candidatelist HmvpCandList (step S2146 of FIG. 28 ). Here, the end of thehistory-based motion vector predictor candidate list is aNumHmvpCand^(th) element HmvpCandList[NumHmvpCand] when counted from a0^(th) element. Also, NumHmvpCand is incremented by 1 and the presentprocess of shifting and adding an element of the history-based motionvector predictor candidate list HmvpCandList ends.

Here, although the history-based motion vector predictor candidate listis applied to both the motion vector predictor mode and the merge mode,the history-based motion vector predictor candidate list may be appliedto only one of the modes.

As described above, because anew element is added after an identicalelement stored in the history-based motion vector predictor candidatelist is removed when the history-based motion vector predictor candidatelist is updated, there are no overlapping elements in the history-basedmotion vector predictor candidate list and the history-based motionvector predictor candidate list includes all different elements.

FIG. 31 is an explanatory diagram showing an example of a process ofupdating the history-based motion vector predictor candidate list. Whena new element is added to the history-based motion vector predictorcandidate list HmvpCandList in which six elements (inter predictioninformation) have been registered, the elements are compared with thenew inter prediction information in order from a front element of thehistory-based motion vector predictor candidate list HmvpCandList (FIG.31A). If the new element has the same value as a third element HMVP2from the beginning of the history-based motion vector predictorcandidate list HmvpCandList, the element HMVP2 is removed from thehistory-based motion vector predictor candidate list HmvpCandList andsubsequent elements HMVP3 to HMVP5 are shifted forward (copied) one byone, and the new element is added to the end of the history-based motionvector predictor candidate list HmvpCandList (FIG. 31B) to complete theupdate of the history-based motion vector predictor candidate listHmvpCandList (FIG. 31C).

<History-Based Motion Vector Predictor Candidate Derivation Process>

Next, a method of deriving a history-based motion vector predictorcandidate from the history-based motion vector predictor candidate listHmvpCandList which is a processing procedure of step S304 of FIG. 20that is a process common to the history-based motion vector predictorcandidate derivation unit 323 of the normal motion vector predictor modederivation unit 301 of the coding side and the history-based motionvector predictor candidate derivation unit 423 of the normal motionvector predictor mode derivation unit 401 of the decoding side will bedescribed in detail. FIG. 29 is an explanatory flowchart showing ahistory-based motion vector predictor candidate derivation processingprocedure.

When the current number of motion vector predictor candidatesnumCurrMvpCand is larger than or equal to the maximum number of elementsin the motion vector predictor candidate list mvpListLX (here, 2) or avalue of the number of history-based motion vector predictor candidatesNumHmvpCand is 0 (NO in step S2201 of FIG. 29 ), the processing of stepsS2202 to S2208 of FIG. 29 is omitted and the history-based motion vectorpredictor candidate derivation processing procedure ends. When thecurrent number of motion vector predictor candidates numCurrMvpCand issmaller than 2 which is the maximum number of elements of the motionvector predictor candidate list mvpListLX and the value of the number ofhistory-based motion vector predictor candidates NumHmvpCand is largerthan 0 (YES in step S2201 of FIG. 29 ), the processing of steps S2202 toS2208 of FIG. 29 is performed.

Subsequently, the processing of steps S2203 to S2207 of FIG. 29 isiterated until the index i changes from 1 to a smaller value of 4 andthe number of history-based motion vector predictor candidatesnumCheckedHMVPCand (steps S2202 to S2208 of FIG. 29 ). When the currentnumber of motion vector predictor candidates numCurrMvpCand is largerthan or equal to 2 which is the maximum number of elements of the motionvector predictor candidate list mvpListLX (step S2203 of FIG. 29 : NO),the processing of steps S2204 to S2208 in FIG. 29 is omitted and thepresent history-based motion vector predictor candidate derivationprocessing procedure ends. When the current number of motion vectorpredictor candidates numCurrMvpCand is smaller than 2 which is themaximum number of elements of the motion vector predictor candidate listmvpListLX (step S2203 of FIG. 29 : YES), the processing from step S2204of FIG. 29 is performed.

Subsequently, the processing of steps S2205 and S2206 is performed forY=0 and 1 (L0 and L1) (steps S2204 to S2207 of FIG. 29 ). When thecurrent number of motion vector predictor candidates numCurrMvpCand islarger than or equal to 2 which is the maximum number of elements of themotion vector predictor candidate list mvpListLX (step S2205 of FIG. 29: NO), the processing of steps S2206 to S2208 of FIG. 29 is omitted andthe present history-based motion vector predictor candidate derivationprocessing procedure ends. When the current number of motion vectorpredictor candidates numCurrMvpCand is smaller than 2 which is themaximum number of elements of the motion vector predictor candidate listmvpListLX (step S2205 of FIG. 29 : YES), the processing from step S2206of FIG. 29 is performed.

Subsequently, the motion vector of LY of the history-based motion vectorpredictor candidate HmvpCandList [NumHmvpCand−i] is added to anumCurrMvpCand^(th) element mvpListLY[numCurrMvpCand] when counted from0 in the motion vector predictor candidate list of LY-prediction and thecurrent number of motion vector predictor candidates numCurrMvpCand isincremented by 1 (step S2206 of FIG. 29 ).

The above processing of steps S2205 and S2206 of FIG. 29 is performedfor both L0 and L1 (steps S2204 to S2207 of FIG. 29 ). When the index iis incremented by 1 and the index i is less than or equal to a smallervalue of 4 and the number of history-based motion vector predictorcandidates NumHmvpCand, the processing from step S2203 is performedagain (steps S2202 to S2208 of FIG. 29 ).

In the present embodiment, as described above, in the history-basedmotion vector predictor candidate derivation process, the motion vectorof the element of the history-based motion vector predictor candidatelist is added to the motion vector predictor candidate list withoutmaking a comparison between the motion vector of the element of thehistory-based motion vector predictor candidate list and the motionvector of the element of the motion vector predictor candidate list.

By adopting this configuration, if the number of history-based motionvector predictor candidates is larger than or equal to 2, it is possibleto ensure that the number of elements of the motion vector predictorcandidate list reaches the maximum number of elements after thehistory-based motion vector predictor candidate list candidatederivation process is completed. Also, an amount of processing and acircuit scale for checking whether or not the motion vectors are thesame can be reduced.

Because the normal motion vector predictor mode is a mode in which themotion information of a target block is determined according to a motionvector difference from a motion vector predictor candidate included inthe motion vector predictor candidate list and an appropriate motionvector is allowed to be determined according to the motion vectordifference, the deterioration of coding efficiency due to the reductionof options can be minimized even if the elements of the motion vectorpredictor candidate list are forced to overlap.

Also, in the normal motion vector predictor mode, the motion vectorpredictor candidate list is processed separately for L0-prediction andL1-prediction. Thus, even if the elements overlap in the motion vectorpredictor candidate list of L0-prediction, the elements may not overlapin the motion vector predictor candidate list of L1-prediction.

Also, the motion vector predictor candidate and the reference index areseparately processed in the normal motion vector predictor mode. Thus,even if the elements overlap in the motion vector predictor candidatelist, the reference index is not affected.

Also, because the motion vector predictor candidate list includes onlytwo elements at the maximum, the number of options to be reduced is onlyone even if the elements of the motion vector predictor candidate listoverlap.

Also, because no identical element is included in the history-basedmotion vector predictor candidate list, a practically meaningfulcomparison between an element of the history-based motion vectorpredictor candidate list and an element of the motion vector predictorcandidate list is limited to case in which only one element of themotion vector predictor candidate list is generated by the spatialmotion vector predictor candidate derivation unit 421 and the temporalmotion vector predictor candidate derivation unit 422 and a frequencythereof is significantly low. Also, a case in which the normal motionvector predictor mode is generally selected is a case in which themotion is not similar to that of neighboring blocks and a possibilitythat an element of the history-based motion vector predictor candidatelist will overlap an element already added to the motion vectorpredictor candidate list is low.

For this reason, even if the elements of the motion vector predictorcandidate list overlap, it is possible to reduce a comparison processbetween the motion vector of the history-based motion vector predictorcandidate and the motion vector of the motion vector predictor candidatewhile limiting the deterioration of coding efficiency due to a decreasein the number of options.

Also, the history-based motion vector predictor candidate list is filledwith non-overlapping history-based motion vector predictor candidates, acomparison between the elements of the history-based motion vectorpredictor candidate list and the elements of the motion vector predictorcandidate list is not made, and an element of the history-based motionvector predictor candidate list is added to the motion vector predictorcandidate list, so that it is possible to omit the process of the motionvector predictor replenishment unit 325 after the history-based motionvector predictor candidate derivation unit 323 in the normal motionvector predictor mode derivation unit 301.

<History-Based Merging Candidate Derivation Process>

Next, a method of deriving history-based merging candidates from thehistory-based merging candidate list HmvpCandList which is theprocessing procedure of step S404 of FIG. 21 which is a process commonto the history-based merging candidate derivation unit 345 of the normalmerge mode derivation unit 302 of the coding side and the history-basedmerging candidate derivation unit 445 of the normal merge modederivation unit 402 of the decoding side will be described in detail.FIG. 30 is an explanatory flowchart showing the history-based mergingcandidate derivation processing procedure.

First, an initialization process is performed (step S2301 of FIG. 30 ).Each (numCurrMergeCand−1)^(th) element from 0 of isPruned[i] is set to avalue of FALSE and a variable numOrigMergeCand is set to the number ofelements numCurrMergeCand registered in the current merging candidatelist.

Subsequently, the initial value of the index hMvpIdx is set to 1 and theaddition process of steps S2303 to S2310 of FIG. 30 is iterated untilthe index hMvpIdx changes from the initial value to NumHmvpCand (stepsS2302 to S2311 of FIG. 30 ). If the number of elements registered in thecurrent merging candidate list numCurrMergeCand is not less than orequal to (the maximum number of merging candidates MaxNumMergeCand−1),merging candidates are added to all elements of the merging candidatelist, so that the present history-based merging candidate derivationprocess ends (NO in step S2303 of FIG. 30 ). When the number of theelements numCurrMergeCand registered in the current merging candidatelist is less than or equal to (the maximum number of merging candidatesMaxNumMergeCand−1), the processing from step S2304 is performed.sameMotion is set to a value of FALSE (step S2304 of FIG. 30 ).Subsequently, the initial value of the index i is set to 0 and theprocessing of steps S2306 and S2307 of FIG. 30 is performed until theindex changes from the initial value to numOrigMergeCand−1 (S2305 toS2308 in FIG. 30 ). A comparison is made regarding whether or not a(NumHmvpCand-hMvpIdx)^(th) element HmvpCandList[NumHmvpCand-hMvpIdx]when counted from a 0^(th) element of the history-based motion vectorprediction candidate list has the same value as an i^(th) elementmergeCandList[i] when counted from a 0^(th) element of a mergingcandidate list (step S2306 of FIG. 30 ).

The merging candidates have the same value when values of all components(an inter prediction mode, a reference index, and a motion vector) ofthe merging candidates are identical. When the merging candidates havethe same value and isPruned[i] is FALSE (YES in step S2306 of FIG. 30 ),both sameMotion and isPruned [i] are set to TRUE (step S2307 of FIG. 30). When the merging candidates do not have the same value (NO in stepS2306 of FIG. 30 ), the processing of step S2307 is skipped. When theiterative processing of steps S2305 to S2308 of FIG. 30 has beencompleted, a comparison is made regarding whether or not sameMotion isFALSE (step S2309 of FIG. 30 ). If sameMotion is FALSE (YES in stepS2309 of FIG. 30 ), i.e., because a (NumHmvpCand-hMvpIdx)^(th) elementHmvpCandList[NumHmvpCand-hMvpIdx] when counted from a 0^(th) element ofthe history-based motion vector predictor candidate list does not existin mergeCandList, a (NumHmvpCand-hMvpIdx)^(th) elementHmvpCandList[NumHmvpCand-hMvpIdx] when counted from a 0^(th) element ofthe history-based motion vector predictor candidate list is added to anumCurrMergeCand^(th) element mergeCandList[numCurrMergeCand] of themerging candidate list and numCurrMergeCand is incremented by 1 (stepS2310 of FIG. 30 ). The index hMvpIdx is incremented by 1 (step S2302 ofFIG. 30 ) and a process of iterating steps S2302 to S2311 of FIG. 30 isperformed.

When the checking of all elements of the history-based motion vectorpredictor candidate list is completed or when merging candidates areadded to all elements of the merging candidate list, the presenthistory-based merging candidate derivation process is completed.

In the present embodiment, as described above, in the history-basedmerging candidate derivation process, the elements of the history-basedmotion vector predictor candidate list are compared with the elements ofthe current merging candidate list and only an element of thehistory-based motion vector predictor candidate list absent in thecurrent merging candidate list is added to the merging candidate list.

Unlike the normal motion vector predictor mode, the normal merge mode isa mode in which the motion information of the target block is directlydetermined without using the motion vector difference. Therefore, it ispossible to improve the coding efficiency by prohibiting the addition ofan element of the history-based merging candidate list overlapping anelement of the current merging candidate list. Here, although theelements of the history-based motion vector predictor candidate listhave been compared with all candidates of the current merging candidatelist are compared, the present invention is not limited thereto as longas it is possible to improve the coding efficiency by comparing at leastthe elements of the history-based motion vector predictor candidate listwith the elements of the current merging candidate list.

For example, the number of elements of the history-based motion vectorpredictor candidate list to be compared may be limited to 1, 2, or thelike. Also, the number of elements of the current merging candidate listto be compared may be limited to 1 or 2.

Also, in the normal merge mode, the merging candidate list includes boththe motion vector of L0-prediction and the motion vector ofL1-prediction. Thus, as in the normal motion vector predictor mode, itis difficult to separately adjust the motion vector predictor ofL0-prediction and the motion vector predictor of L1-prediction.

Also, in the normal merge mode, the merging candidate list includes boththe reference index for L0-prediction and the reference index forL1-prediction. Thus, as in the normal motion vector predictor mode, itis difficult to separately adjust the reference index of theL0-prediction and the reference index of the L1-prediction.

Also, because the merging candidate list includes a maximum of 6elements, which are more than those of the motion vector predictorcandidate list, the number of overlapping elements within the mergingcandidate list increases and it is difficult to efficiently use themerging candidate list when an overlapping element has been added to themerging candidate list.

Also, the element of the history-based motion vector predictor candidatelist added to the merging candidate list is the most recently addedelement among the elements of the history-based motion vector predictorcandidate list. Thus, the elements of the history-based motion vectorpredictor candidate list added to the merging candidate list becomemotion information closest to a target coding block in the space domain.In general, the normal merge mode is selected when motion is similar tothat of neighboring blocks and a possibility that the elements of thehistory-based motion vector predictor candidate list will overlap theelements already added to the merging candidate list becomes high.

It is possible to improve the coding efficiency by prohibiting theaddition of an element of the history-based motion vector predictorcandidate list overlapping an element of the merging candidate list andincreasing the number of valid selection elements in order to solve theproblems as described above.

Also, the selection probability of the normal merge mode increasesbecause 6 which is the maximum number of elements capable of beingincluded in the merging candidate list is larger than 2 which is themaximum number of elements capable of being included in the motionvector predictor candidate list, so that it is possible to reduce thecomparison process between the motion vector of the history-based motionvector predictor candidate and the motion vector of the motion vectorpredictor candidate while limiting the deterioration of codingefficiency due to the overlapping of the elements of the motion vectorpredictor candidate list.

Also, various merging candidates such as temporal merging candidates,history-based merging candidates, average merging candidates, and zeromerging candidates are included in addition to spatial mergingcandidates and the selection probability of the normal merge modeincreases, so that it is possible to reduce the comparison processbetween the motion vector of the history-based motion vector predictorcandidate and the motion vector of the motion vector predictor candidatewhile limiting the deterioration of coding efficiency due to theoverlapping of the elements of the motion vector predictor candidatelist.

<Motion-Compensated Prediction Process>

The motion-compensated prediction unit 306 acquires a position and asize of a block that is a current target of a prediction process incoding. Also, the motion-compensated prediction unit 306 acquires interprediction information from the inter prediction mode determination unit305. A reference index and a motion vector are derived from the acquiredinter prediction information and a prediction signal is generated aftera picture signal of a position to which a reference picture identifiedby the reference index within the decoded picture memory 104 is movedfrom a position identical to that of a picture signal of a predictionblock by an amount of motion vector is acquired.

A motion-compensated prediction signal is supplied to a predictionmethod determination unit 105 using a prediction signal acquired fromone reference picture as a motion-compensated prediction signal when theinter prediction mode in the inter prediction is prediction from asingle reference picture such as L0-prediction or L1-prediction andusing a prediction signal obtained by weighted-averaging predictionsignals acquired from two reference pictures as a motion-compensatedprediction signal when the prediction mode is prediction from tworeference pictures such as an inter prediction mode of Bi-prediction.Although a weighted average ratio of bi-prediction is 1:1 here, aweighted average may be performed using another ratio. For example, aweighting ratio may increase as the picture interval between a picture,which is a prediction target, and a reference picture decreases. Also,the weighting ratio may be calculated using a corresponding tablebetween combinations of picture intervals and weighting ratios.

The motion-compensated prediction unit 406 has a function similar tothat of the motion-compensated prediction unit 306 of the coding side.The motion-compensated prediction unit 406 acquires inter predictioninformation from the normal motion vector predictor mode derivation unit401, the normal merge mode derivation unit 402, the subblock-basedmotion vector predictor mode derivation unit 403, and the subblock-basedmerge mode derivation unit 404 via the switch 408. Themotion-compensated prediction unit 406 supplies an obtainedmotion-compensated prediction signal to the decoding picture signalsuperimposition unit 207.

<About Inter Prediction Mode>

A process of performing prediction from a single reference picture isdefined as uni-prediction. In the case of uni-prediction, predictionusing either one of two reference pictures registered in reference listsL0 and L1 such as L0-prediction or L1-prediction is performed.

FIG. 32 shows the case of uni-prediction in which a clock time of areference picture (RefL0Pic) of L0 is earlier than that of a targetpicture (CurPic). FIG. 33 shows the case of uni-prediction in which aclock time of a reference picture of the L0-prediction is later thanthat of a target picture. Likewise, the reference picture ofL0-prediction of FIGS. 32 and 33 can be replaced with a referencepicture (RefL1Pic) of L1-prediction to perform uni-prediction.

The process of performing prediction from two reference pictures isdefined as bi-prediction and the bi-prediction is represented asBi-prediction using both L0-prediction and L1-prediction. FIG. 34 showsthe case of the bi-prediction in which a clock time of a referencepicture of L0-prediction is earlier than that of a target picture and aclock time of a reference picture of L1-prediction is later than that ofthe target picture. FIG. 35 shows the case of bi-prediction in whichclock times of the reference picture of L0-prediction and the referencepicture of L1-prediction are earlier than that of a target picture. FIG.36 shows the case of bi-prediction in which a clock time of a referencepicture of L0-prediction and a clock time of a reference picture ofL1-prediction are later than that of a target picture.

As described above, a relationship between a type of prediction of L0/L1and time can be used without being limited to L0 which is in the pastdirection and L1 which is in the future direction. In the case ofbi-prediction, each of L0-prediction and L1-prediction may be performedusing the same reference picture. Also, it is determined whether toperform motion-compensated prediction according to uni-prediction orbi-prediction on the basis of, for example, information (for example, aflag) indicating whether to use L0-prediction and whether to useL1-prediction.

<About Reference Index>

In the embodiment of the present invention, it is possible to select anoptimum reference picture from a plurality of reference pictures inmotion-compensated prediction to improve the accuracy ofmotion-compensated prediction. Thus, the reference picture used in themotion-compensated prediction is used as a reference index and thereference index is coded in the bitstream together with the motionvector difference.

<Motion Compensation Process Based on Normal Motion Vector PredictorMode>

As shown in the inter prediction unit 102 of the coding side of FIG. 16, when inter prediction information from the normal motion vectorpredictor mode derivation unit 301 has been selected in the interprediction mode determination unit 305, the motion-compensatedprediction unit 306 acquires the inter prediction information from theinter prediction mode determination unit 305, derives an interprediction mode, a reference index, and a motion vector of a currenttarget block, and generates a motion-compensated prediction signal. Thegenerated motion-compensated prediction signal is supplied to theprediction method determination unit 105.

Likewise, as shown in the inter prediction unit 203 of the decoding sideof FIG. 22 , when the switch 408 has been connected to the normal motionvector predictor mode derivation unit 401 in the decoding process, themotion-compensated prediction unit 406 acquires inter predictioninformation from the normal motion vector predictor mode derivation unit401, derives an inter prediction mode, a reference index, and a motionvector of a current target block, and generates a motion-compensatedprediction signal. The generated motion-compensated prediction signal issupplied to the decoding picture signal superimposition unit 207.

<Motion Compensation Process Based on Normal Merge Mode>

Also, as shown in the inter prediction unit 102 in the coding side ofFIG. 16 , when inter prediction information has been selected from thenormal merge mode derivation unit 302 in the inter prediction modedetermination unit 305, the motion-compensated prediction unit 306acquires the inter prediction information from the inter prediction modedetermination unit 305, derives an inter prediction mode, a referenceindex, and a motion vector of a current target block, and generates amotion-compensated prediction signal. The generated motion-compensatedprediction signal is supplied to the prediction method determinationunit 105.

Likewise, as shown in the inter prediction unit 203 in the decoding sideof FIG. 22 , when the switch 408 has been connected to the normal mergemode derivation unit 402 in the decoding process, the motion-compensatedprediction unit 406 acquires inter prediction information from thenormal merge mode derivation unit 402, derives an inter prediction mode,a reference index, and a motion vector of a current target block, andgenerates a motion-compensated prediction signal. The generatedmotion-compensated prediction signal is supplied to the decoding picturesignal superimposition unit 207.

<Motion Compensation Process Based on Subblock-Based Motion VectorPredictor Mode>

Also, as shown in the inter prediction unit 102 on the coding side ofFIG. 16 , when inter prediction information from the subblock-basedmotion vector predictor mode derivation unit 303 has been selected inthe inter prediction mode determination unit 305, the motion-compensatedprediction unit 306 acquires the inter prediction information from theinter prediction mode determination unit 305, derives an interprediction mode, a reference index, and a motion vector of a currenttarget block, and generates a motion-compensated prediction signal. Thegenerated motion-compensated prediction signal is supplied to theprediction method determination unit 105.

Likewise, as shown in the inter prediction unit 203 in the decoding sideof FIG. 22 , when the switch 408 has been connected to thesubblock-based motion vector predictor mode derivation unit 403 in thedecoding process, the motion-compensated prediction unit 406 acquiresinter prediction information from the subblock-based motion vectorpredictor mode derivation unit 403, derives an inter prediction mode, areference index, and a motion vector of a current target block, andgenerates a motion-compensated prediction signal. The generatedmotion-compensated prediction signal is supplied to the decoding picturesignal superimposition unit 207.

<Motion Compensation Process Based on Subblock-Based Merge Mode>

Also, as shown in the inter prediction unit 102 on the coding side ofFIG. 16 , when inter prediction information from the subblock-basedmerge mode derivation unit 304 has been selected in the inter predictionmode determination unit 305, the motion-compensated prediction unit 306acquires the inter prediction information from the inter prediction modedetermination unit 305, derives an inter prediction mode, a referenceindex, and a motion vector of a current target block, and generates amotion-compensated prediction signal. The generated motion-compensatedprediction signal is supplied to the prediction method determinationunit 105.

Likewise, as shown in the inter prediction unit 203 in the decoding sideof FIG. 22 , when the switch 408 has been connected to thesubblock-based merge mode derivation unit 404 in the decoding process,the motion-compensated prediction unit 406 acquires inter predictioninformation from the subblock-based merge mode derivation unit 404,derives an inter prediction mode, a reference index, and a motion vectorof a current target block, and generates a motion-compensated predictionsignal. The generated motion-compensated prediction signal is suppliedto the decoding picture signal superimposition unit 207.

<Motion Compensation Process Based on Affine Transform Prediction>

In the normal motion vector predictor mode and the normal merge mode,motion compensation of an affine model can be used on the basis of thefollowing flags. The following flags are reflected in the followingflags on the basis of inter prediction conditions determined by theinter prediction mode determination unit 305 in the coding process andare coded in a bitstream. In the decoding process, it is identifiedwhether to perform the motion compensation of the affine model on thebasis of the following flags in the bitstream.

sps_affine_enabled_flag represents whether or not motion compensation ofthe affine model can be used in inter prediction. Ifsps_affine_enabled_flag is 0, suppression is performed so that it is notmotion compensation of an affine model in units of sequences. Also,inter_affine_flag and cu_affine_type_flag are not transmitted in CU(coding block) syntax of a coding video sequence. Ifsps_affine_enabled_flag is 1, motion compensation of an affine model canbe used in a coding video sequence.

sps_affine_type_flag represents whether or not motion compensation of asix-parameter affine model can be used in inter prediction. Ifsps_affine_type_flag is 0, suppression is performed so that it is notmotion compensation of the six-parameter affine model. Also,cu_affine_type_flag is not transmitted in CU syntax of a coding videosequence. If sps_affine_type_flag is 1, motion compensation of thesix-parameter affine model can be used in the coding video sequence.When sps_affine_type_flag does not exist, it is assumed to be 0.

When a P or B slice is decoded, if inter_affine_flag is 1 in the currenttarget CU, motion compensation of the affine model is used to generate amotion-compensated prediction signal of the current target CU. Ifinter_affine_flag is 0, the affine model is not used in the currenttarget CU. When inter_affine_flag does not exist, it is assumed to be 0.

When a P or B slice is decoded, if cu_affine_type_flag is 1 in thecurrent target CU, motion compensation of a six-parameter affine modelis used to generate a motion-compensated prediction signal of thecurrent target CU. If cu_affine_type_flag is 0, motion compensation of afour-parameter affine model is used to generate a motion-compensatedprediction signal of the current target CU.

In motion compensation of an affine model, because a reference index anda motion vector are derived in units of subblocks, a motion-compensatedprediction signal is generated using a reference index or a motionvector which is a target in units of subblocks.

A four-parameter affine model is a mode in which the motion vector ofthe subblock is derived from four parameters of horizontal componentsand vertical components of motion vectors of the two control points andmotion compensation is performed in units of subblocks.

Modified Example 1

Modified example 1 of the present embodiment will be described. Thepresent modified example is different from the present embodiment inthat a merge motion vector difference mode is added. Only differencesfrom the present embodiment will be described. The merge motion vectordifference mode is set if umve_flag in FIG. 12 is 1 and the normal mergemode is set if umve_flag is 0.

Next, an operation of the merge motion vector difference mode will bedescribed. The merge motion vector difference mode is a mode in whichone merge motion vector difference can be added to each of a motionvector of L0-prediction and a motion vector of L1-prediction in onemerging candidate of two higher-order merging candidates (mergingcandidates having merge indices of 0 and 1 within a merging candidatelist).

Also, in the case of the merge motion vector difference mode, the mergemotion vector difference is coded by the bit strings coding unit 108 anddecoded by the bit strings decoding unit 201.

As described above, because the merging candidate list is also used inthe merge motion vector difference mode, it is possible to improve thecoding efficiency by prohibiting the addition of an element of thehistory-based motion vector predictor candidate list overlapping anelement of the merging candidate list and increasing the number of validselection elements.

Also, the selection probability of the normal motion vector predictormode is decreased by adding the merge motion vector difference mode, sothat it is possible to reduce the comparison process between the motionvector of the history-based motion vector predictor candidate and themotion vector of the motion vector predictor candidate while limitingthe deterioration of coding efficiency due to the overlapping of theelements of the motion vector predictor candidate list.

Modified Example 2

Modified example 2 of the present embodiment will be described. Thepresent modified example is different from the present embodiment inthat the operations of the history-based motion vector predictorcandidate derivation units 323 and 423 shown in FIG. 29 are differentfrom an operation of the present embodiment. Only differences from thepresent embodiment will be described. FIG. 41 is a flowchartillustrating a history-based motion vector predictor candidatederivation processing procedure of modified example 2. FIG. 41 isdifferent from FIG. 29 in that step S2209 is added. The operation shownin FIG. 29 is the same as that shown in FIG. 41 , except for step S2209of FIG. 41 .

A process when numCurrMvpCand is smaller than 2, which is the maximumnumber of elements of the motion vector predictor candidate list (YES instep S2205 of FIG. 41 ) will be described.

It is checked whether or not the reference index of LY of thehistory-based motion vector predictor candidateHmvpCandList[NumHmvpCand−i] is the same as the reference index of LY ofa target coding block (step S2209 of FIG. 41 ). If the reference indexof LY of the history-based motion vector predictor candidateHmvpCandList[NumHmvpCand−i] is the same as the reference index of LY ofthe target coding block (YES in step S2209 of FIG. 41 ), the processproceeds to S2206. If the reference index of LY of the history-basedmotion vector predictor candidate HmvpCandList[NumHmvpCand−i] is not thesame as the reference index of LY of the target coding block (NO in stepS2209 of FIG. 41 ), the process proceeds to S2207.

As described above, when the reference index of LY of the history-basedmotion vector predictor candidate is the same as the reference index ofLY of the target coding block, the motion vector of the history-basedmotion vector predictor candidate is added to the motion vectorpredictor candidate list of LY-prediction, so that it is possible to addhighly accurate history-based motion vector predictor candidates to themotion vector predictor candidate list without comparing the motionvector of the history-based motion vector predictor candidate with themotion vector of the motion vector predictor candidate.

In the present embodiment, an element of the history-based motion vectorpredictor candidate list is added to the motion vector predictorcandidate list without comparing the elements of the history-basedmotion vector predictor candidate list with the elements of the motionvector predictor candidate list in the history-based motion vectorpredictor candidate derivation process, whereas the elements of thehistory-based merging candidate list are compared with the elements ofthe merging list and only a history-based merging candidate list absentin the merging list is added to the merging list in the history-basedmerging candidate derivation process. The following effects can beobtained by adopting the above configuration.

1. In the history-based motion vector predictor candidate list candidatederivation process, it is not necessary to perform an additionalcandidate derivation process, it is possible to reduce an amount ofprocessing amount and a circuit scale, and it is possible to minimizethe deterioration of coding efficiency due to the reduction of optionseven if the elements of the motion vector predictor candidate list areforced to overlap.

2. In the history-based merging candidate derivation process, it ispossible to construct an appropriate merging candidate list and improvecoding efficiency in the normal merge mode in which the motioninformation of the target block is determined without using the motionvector difference by prohibiting the addition of an element of thehistory-based merging candidate list overlapping an element of the mergelist.

3. It is possible to fill the history-based motion vector predictorcandidate list with non-overlapping history-based motion vectorpredictor candidates and it is possible to omit a process of the mergingcandidate replenishment unit 346 after the history-based mergingcandidate derivation unit 345 in the normal merge mode derivation unit302 and reduce an amount of processing.

4. The history-based motion vector predictor candidate list is filledwith non-overlapping history-based motion vector predictor candidates, acomparison between the elements of the history-based motion vectorpredictor candidate list and the elements of the motion vector predictorcandidate list is not made, and an element of the history-based motionvector predictor candidate list is added to the motion vector predictorcandidate list, so that it is possible to omit the process of the motionvector predictor replenishment unit 325 after the history-based motionvector predictor candidate derivation unit 323 in the normal motionvector predictor mode derivation unit 301.

Two or more of all the embodiments described above may be combined.

In all the embodiments described above, a bitstream output by thepicture coding device has a specific data format so that the bitstreamcan be decoded in accordance with the coding method used in theembodiment. Also, a picture decoding device corresponding to the picturecoding device can decode the bitstream of the specific data format.

When a wired or wireless network is used to exchange a bitstream betweenthe picture coding device and the picture decoding device, the bitstreammay be converted into a data format suitable for a transmission form ofa communication path and transmitted. In this case, a transmissiondevice for converting the bitstream output from the picture codingdevice into coded data of a data format suitable for the transmissionform of the communication path and transmitting the coded data to thenetwork and a reception device for receiving the coded data from thenetwork, restoring the coded data to the bitstream, and supplying thebitstream to the picture decoding device are provided. The transmissiondevice includes a memory that buffers the bitstream output by thepicture coding device, a packet processing unit that packetizes thebitstream, and a transmission unit that transmits packetized coded datavia the network. The reception device includes a reception unit thatreceives the packetized coded data via the network, a memory thatbuffers the received coded data, and a packet processing unit thatgenerates a bitstream by performing packet processing on the coded dataand supplies the bitstream to the picture decoding device.

Also, a display device may be provided by adding a display unit thatdisplays a picture decoded by the picture decoding device to theconfiguration. In this case, the display unit reads a decoded picturesignal generated by the decoding picture signal superimposition unit 207and stored in the decoded picture memory 208 and displays the decodedpicture signal on a screen.

Also, an imaging device may be provided by adding an imaging unit thatinputs a captured picture to the picture coding device to theconfiguration. In this case, the imaging unit inputs a captured picturesignal to the block split unit 101.

FIG. 37 shows an example of a hardware configuration of thecoding/decoding device according to the present embodiment. Thecoding/decoding device includes the configuration of the picture codingdevice and the picture decoding device according to the embodiment ofthe present invention. A related coding/decoding device 9000 includes aCPU 9001, a codec IC 9002, an I/O interface 9003, a memory 9004, anoptical disc drive 9005, a network interface 9006, and a video interface9009 and the respective parts are connected by a bus 9010.

A picture coding unit 9007 and a picture decoding unit 9008 aretypically implemented as the codec IC 9002. A picture coding process ofthe picture coding device according to the embodiment of the presentinvention is executed by the picture coding unit 9007 and a picturedecoding process in the picture decoding device according to theembodiment of the present invention is performed by the picture decodingunit 9008. The I/O interface 9003 is implemented by, for example, a USBinterface, and is connected to an external keyboard 9104, a mouse 9105,and the like. The CPU 9001 controls the coding/decoding device 9000 sothat a user-desired operation is executed on the basis of a useroperation input via the I/O interface 9003. User operations using thekeyboard 9104, the mouse 9105, and the like include the selection of acoding or decoding function to be executed, setting of coding quality,designation of an input/output destination of a bitstream, designationof an input/output destination of a picture, and the like.

When the user desires an operation of reproducing a picture recorded ona disc recording medium 9100, the optical disc drive 9005 reads abitstream from the disc recording medium 9100 that has been inserted andtransmits the read bitstream to the picture decoding unit 9008 of thecodec IC 9002 via the bus 9010. The picture decoding unit 9008 executesa picture decoding process on the input bitstream in the picturedecoding device according to the embodiment of the present invention andtransmits a decoded picture to an external monitor 9103 via the videointerface 9009. The coding/decoding device 9000 includes a networkinterface 9006 and can be connected to an external distribution server9106 and a portable terminal 9107 via a network 9101. When the userdesires to reproduce the picture recorded on the distribution server9106 or the portable terminal 9107 instead of the picture recorded onthe disc recording medium 9100, the network interface 9006 acquires abitstream from the network 9101 instead of reading the bitstream fromthe input disc recording medium 9100. When the user desires to reproducethe picture recorded in the memory 9004, the picture decoding process inthe picture decoding device according to the embodiment of the presentinvention is executed on the bitstream recorded in the memory 9004.

When the user desires to perform an operation of coding a picturecaptured by the external camera 9102 and recording the coded picture inthe memory 9004, the video interface 9009 inputs the picture from thecamera 9102 and transmits the picture to the picture coding unit 9007 ofthe codec IC 9002 via the bus 9010. The picture coding unit 9007executes a picture coding process on a picture input via the videointerface 9009 in the picture coding device according to the embodimentof the present invention to create a bitstream. Then, the bitstream istransmitted to the memory 9004 via the bus 9010. When the user desiresto record a bitstream on the disc recording medium 9100 instead of thememory 9004, the optical disc drive 9005 writes the bitstream to thedisc recording medium 9100 which has been inserted.

It is also possible to implement a hardware configuration that includesa picture coding device without including a picture decoding device or ahardware configuration that includes a picture decoding device withoutincluding a picture coding device. Such a hardware configuration isimplemented, for example, by replacing the codec IC 9002 with thepicture coding unit 9007 or the picture decoding unit 9008.

The above processes related to coding and decoding may be implemented asa transmission, storage, and reception device using hardware andimplemented by firmware stored in a read only memory (ROM), a flashmemory, or the like or software of a computer or the like. A firmwareprogram and a software program thereof may be provided by recording theprograms on a recording medium capable of being read by a computer orthe like or may be provided from a server through a wired or wirelessnetwork or may be provided as data broadcasts of terrestrial orsatellite digital broadcasting.

The present invention has been described above on the basis of theembodiments. The embodiments are examples and it will be understood bythose skilled in the art that various modifications are possible incombinations of the respective components and processing processes andsuch modifications are within the scope of the present invention.

EXPLANATION OF REFERENCES

-   -   100 Picture coding device    -   101 Block split unit    -   102 Inter prediction unit    -   103 Intra prediction unit    -   104 Decoded picture memory    -   105 Prediction method determination unit    -   106 Residual generation unit    -   107 Orthogonal transform/quantization unit    -   108 Bit strings coding unit    -   109 Inverse quantization/inverse orthogonal transform unit    -   110 Decoding picture signal superimposition unit    -   111 Coding information storage memory    -   200 Picture decoding device    -   201 Bit strings decoding unit    -   202 Block split unit    -   203 Inter prediction unit    -   204 Intra prediction unit    -   205 Coding information storage memory    -   206 Inverse quantization/inverse orthogonal transform unit    -   207 Decoding picture signal superimposition unit    -   208 Decoded picture memory

What is claimed is:
 1. A picture decoding device comprising: a spatialcandidate derivation unit configured to derive a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and register the derived spatial candidate as a candidate to afirst candidate list; a history-based candidate derivation unitconfigured to generate a second candidate list by adding a history-basedcandidate included in a history-based candidate list as a candidate tothe first candidate list; a candidate selection unit configured toselect a selection candidate from candidates included in the secondcandidate list; and an inter prediction unit configured to perform interprediction using the selection candidate, wherein the history-basedcandidate derivation unit switches between whether or not ahistory-based candidate overlapping a candidate included in the firstcandidate list is added in accordance with a prediction mode, whereinthe prediction mode is a merge mode and a motion vector predictor mode,the candidate, when the prediction mode is the merge mode, is motioninformation, and the candidate, when the prediction mode is the motionvector predictor mode, is a motion vector, and wherein the history-basedcandidate is added as a candidate to the first candidate list regardlessof whether or not the history-based candidate overlaps a candidateincluded in the first candidate list if a reference index of thehistory-based candidate is the same as a reference index of the decodingtarget picture when the prediction mode is the motion vector predictormode.
 2. A picture decoding method for a picture decoding device, thepicture decoding method comprising: deriving a spatial candidate frominter prediction information of a block neighboring a decoding targetblock and registering the derived spatial candidate as a candidate to afirst candidate list; generating a second candidate list by adding ahistory-based candidate included in a history-based candidate list as acandidate to the first candidate list and switching between whether ornot a history-based candidate overlapping a candidate included in thefirst candidate list is added in accordance with a prediction mode;selecting a selection candidate from candidates included in the secondcandidate list; and performing inter prediction using the selectioncandidate, wherein the prediction mode is a merge mode and a motionvector predictor mode, the candidate, when the prediction mode is themerge mode, is motion information, and the candidate, when theprediction mode is the motion vector predictor mode, is a motion vector,and wherein the history-based candidate is added as a candidate to thefirst candidate list regardless of whether or not the history-basedcandidate overlaps a candidate included in the first candidate list if areference index of the history-based candidate is the same as areference index of the decoding target picture when the prediction modeis the motion vector predictor mode.
 3. A computer program stored in acomputer-readable non-transitory storage medium in a picture decodingdevice, the computer program including instructions for causing acomputer of the picture decoding device to execute: deriving a spatialcandidate from inter prediction information of a block neighboring adecoding target block and registering the derived spatial candidate as acandidate to a first candidate list; generating a second candidate listby adding a history-based candidate included in a history-basedcandidate list as a candidate to the first candidate list and switchingbetween whether or not a history-based candidate overlapping a candidateincluded in the first candidate list is added in accordance with aprediction mode; selecting a selection candidate from candidatesincluded in the second candidate list; and performing inter predictionusing the selection candidate, wherein the prediction mode is a mergemode and a motion vector predictor mode, the candidate, when theprediction mode is the merge mode, is motion information, and thecandidate, when the prediction mode is the motion vector predictor mode,is a motion vector, and wherein the history-based candidate is added asa candidate to the first candidate list regardless of whether or not thehistory-based candidate overlaps a candidate included in the firstcandidate list if a reference index of the history-based candidate isthe same as a reference index of the decoding target picture when theprediction mode is the motion vector predictor mode.
 4. A picture codingdevice comprising: a spatial candidate derivation unit configured toderive a spatial candidate from inter prediction information of a blockneighboring a coding target block and register the derived spatialcandidate as a candidate to a first candidate list; a history-basedcandidate derivation unit configured to generate a second candidate listby adding a history-based candidate included in a history-basedcandidate list as a candidate to the first candidate list; a candidateselection unit configured to select a selection candidate fromcandidates included in the second candidate list; and an interprediction unit configured to perform inter prediction using theselection candidate, wherein the history-based candidate derivation unitswitches between whether or not a history-based candidate overlapping acandidate included in the first candidate list is added in accordancewith a prediction mode, wherein the prediction mode is a merge mode anda motion vector predictor mode, the candidate, when the prediction modeis the merge mode, is motion information, and the candidate, when theprediction mode is the motion vector predictor mode, is a motion vector,and wherein the history-based candidate is added as a candidate to thefirst candidate list regardless of whether or not the history-basedcandidate overlaps a candidate included in the first candidate list if areference index of the history-based candidate is the same as areference index of the coding target picture when the prediction mode isthe motion vector predictor mode.
 5. A picture coding method comprising:deriving a spatial candidate from inter prediction information of ablock neighboring a coding target block and registering the derivedspatial candidate as a candidate to a first candidate list; generating asecond candidate list by adding a history-based candidate included in ahistory-based candidate list as a candidate to the first candidate listand switching between whether or not a history-based candidateoverlapping a candidate included in the first candidate list is added inaccordance with a prediction mode; selecting a selection candidate fromcandidates included in the second candidate list; and performing interprediction using the selection candidate, wherein the prediction mode isa merge mode and a motion vector predictor mode, the candidate, when theprediction mode is the merge mode, is motion information, and thecandidate, when the prediction mode is the motion vector predictor mode,is a motion vector, and wherein the history-based candidate is added asa candidate to the first candidate list regardless of whether or not thehistory-based candidate overlaps a candidate included in the firstcandidate list if a reference index of the history-based candidate isthe same as a reference index of the coding target picture when theprediction mode is the motion vector predictor mode.
 6. A computerprogram stored in a computer-readable non-transitory storage medium in apicture coding device, the computer program including instructions forcausing a computer of the picture coding device to execute: deriving aspatial candidate from inter prediction information of a blockneighboring a coding target block and registering the derived spatialcandidate as a candidate to a first candidate list; generating a secondcandidate list by adding a history-based candidate included in ahistory-based candidate list as a candidate to the first candidate list,switching between whether or not a history-based candidate overlapping acandidate included in the first candidate list is added in accordancewith a prediction mode; selecting a selection candidate from candidatesincluded in the second candidate list; and performing inter predictionusing the selection candidate, wherein the prediction mode is a mergemode and a motion vector predictor mode, the candidate when theprediction mode is the merge mode is motion information, and thecandidate when the prediction mode is the motion vector predictor modeis a motion vector, and wherein the history-based candidate is added asa candidate to the first candidate list regardless of whether or not thehistory-based candidate overlaps a candidate included in the firstcandidate list if a reference index of the history-based candidate isthe same as a reference index of the coding target picture when theprediction mode is the motion vector predictor mode.